In the Anapanasati Sutta what is meant by 'sensitive to rapture/bliss'? Considering these verses of the Anapanasati Sutta


  He trains himself, 'I will breathe in sensitive to rapture.' He trains
  himself, 'I will breathe out sensitive to rapture.' 
  
  He trains himself, 'I will breathe in sensitive to pleasure.' He
  trains himself, 'I will breathe out sensitive to pleasure.'


In the commentaries I have read rapture and pleasure (bliss) are taken as jhana factors as detailed here. What exactly is been suggested here? Is it possible to experience these factors in meditation without being in the jhanas and maybe encourage them? Can one encourage them by focusing on them and almost willing them into being? I was always told this was counterproductive. Presumably this part of the sutta is samatha. Is it being suggested that you should be in the jhanas before going on any further in this meditation?

I think generally I'm just wanting some additional explanation to what these verses mean and the implication of them to meditation practice.
I assume that by rapture and bliss you mean the Pali words Piti and Sukha.

Piti and Sukha are supposed to be comfortable and pleasant states of mind that arise whenever the mind is sufficiently calm, abiding in itself. However, these states have degrees of maturity, and they are not always conventionally pleasant. Diarrhea or sensations of ants crawling for example are results of Piti that the commoner wouldn't enjoy as pleasures. 

Piti and Sukha can arise both in Samatha and Vipassana, leading some to compare the nanas of Vipassana with the Jhanas of Samatha, and claim they are one and the same with subtle differences. However, this is not agreed upon by all, who think there's a big difference between khanika samadhi and jhana. 

My own view is nanas &amp; jhanas are equally beneficial, but different in character to some extent, yet not as different as the hardliners make it out to be.

However Piti and Sukha are false fruits - one may be captured by them, and remaining content in them, may return to them time and again without making more effort to enter higher nanas or jhanas.



There are five types of Piti with the final kind being the jhana factor Piti. 

One can be practicing Vipassana, and from the final Piti depart to a Samadhi, or exit a jhana samadhi directly into a Vipassana nana.

Five kinds or types of Piti


  1. Khuddaka piti (minor rapture)
  
  This state is characterized by the following:
  
  
  The meditator may be aware of a white color. 
  There may be a feeling of coolness or dizziness and the hairs of the body may stand on end. 
  The meditator may cry or feel terrified.
  
  
  2. Khanika piti (momentary rapture)
  
  Characteristics of this piti include:
  
  
  Seeing flashes of light. 
  Seeing sparks. Nervous twitching.  
  A feeling of stiffness all over the body.  
  A feeling as if ants were crawling on the body.  
  A feeling of heat all over the body.  
  Shivering.  
  Seeing red colors. 
  The hair on the body rising slightly. 
  Itchiness as if ants were crawling on one's face and body.   
  
  
  3. Okkantika piti (flood of joy)
  
  In this piti:
  
  
  The body may shake and tremble.  
  The face, hands and feet may twitch.
  There may be violent shaking as if the bed is going to turn upside
  down.  
  Nausea and at times actual vomiting may occur.  
  There may be a
  rhythmic feeling like waves breaking on the shore.  
  Ripples of energy
  may seem to flow over the body.  
  The body may vibrate like a stick
  which is fixed in a flowing stream.  
  A light yellow color may be
  observed. The body may bend to and fro.      
  
  
  4. Ubbenka piti (uplifting joy)
  
  In this piti:
  
  
  The body feels as if it is extending or moving upwards.  
  There may be a
  feeling as though lice are climbing on the face and body.  
  Diarrhea may
  occur.  
  The body may bend forwards or backwards.  
  One may feel that
  one's head has been moved backwards and forwards by somebody.  
  There
  may be a chewing movement with the mouth either open or closed.  
  The
  body sways like a tree being blown by the wind.  
  The body bends
  forwards and may fall down.  
  There may be fidgeting movements of the
  body.  
  There may be jumping movements of the body.  
  Arms and legs may be
  raised or may twitch.  
  The body may bend forwards or may recline.  
  A
  silver gray color may be observed.
  
  
  5. Pharana piti (pervading rapture)
  
  In this piti:
  
  
  A feeling of coldness spreads through the body.  
  Peace of mind sets in
  occasionally.  
  There may be itchy feelings all over the body.  
  There may
  be drowsy feelings and the meditator may not wish to open his or her
  eyes.  
  The meditator has no wish to move.  
  There may be a flushing
  sensation from feet to head or vice versa.  
  The body may feel cool as
  if taking a bath or touching ice.  
  The meditator may see blue or
  emerald green colors.  
  An itchy feeling as though lice are crawling on
  the face may occur.
  


Sukha


  The fourth defilement of vipassana is sukha which means "bliss" and
  has the following characteristics:
  
  
  There may be a feeling of comfort.   
  Due to pleasant feelings the
  meditator may wish to continue practicing for a long time.   
  The
  meditator may wish to tell other people of the results which he has
  already gained.   
  The meditator may feel immeasurably proud and happy.
  Some say that they have never known such happiness.   
  Some feel deeply
  grateful to their teachers.   
  Some meditators feel that their teacher is
  at hand to give help.
  


Source: http://www.vipassanadhura.com/sixteen.html

One cannot understand the meaning of these terms sufficiently by poring through a Pali dictionary; true understanding in these cases is experiential.

Not everyone attains these states in sequence, one's mind can skip some steps, or one's attention may not catch them when they occur. 

If one is on a retreat, and only practicing Anapanasati for several days, one can certainly come very close to the textbook definitions, but that doesn't make the experience special. What makes the experience special is the several days of sustained practice.


  If all the Buddha's teachings are lost, one can recover all of them by faithfully following the breath.


I instinctively felt this statement arise in me with no doubt whatsoever when I first encountered the Anapanasati Sutta, which was the first Buddhist text of any sort that I had read. I was pleasantly surprised some days back to see the Buddha has said much the same - that one can reach enlightenment with just the Anapanasati practice.

Most monasteries in ancient times only had a few suttas at any one time, and this accounts for the great profusion of texts today which were each written from an individual's personal realization of the grand truths.

The ancient masters who took Buddhism to several distant lands had certainly not read every text there was on the topic - they couldn't have because many of them weren't yet written. Yet, their transmission was pure because their realization was personal - they had walked the same path as the Buddha, and seen the same things. That was enough.

Being flooded with various texts from several traditions today thanks to the Internet, the challenge for each of us is to find time from reading to actually practice. A foundational text like Anapanasati actually contains all the necessary elements of practice. All we need is the confidence to read no more, and lock ourselves away in the practice room.

UPDATE:

I think there are a few specific questions I forgot to answer:     


Is it possible to experience these factors in meditation without being in the jhanas and maybe encourage them? 


Yes

Can one encourage them by focusing on them and almost willing them into being? 


I think this recent Dhamma talk by Thanissaro Bhikkhu, "Don't focus on Jhana, focus on the breath" (mp3 linked) answers that.

I was always told this was counterproductive. 


Usually yes, because meditation becomes a strain, and goal oriented.

Presumably this part of the sutta is samatha. 


It is a core meditation technique for mindfulness and concentration - both. It is mentioned in the Satipatthana sutta as such. 



http://www.accesstoinsight.org/tipitaka/mn/mn.010.than.html


  "There is the case where a monk — having gone to the wilderness, to
  the shade of a tree, or to an empty building — sits down folding his
  legs crosswise, holding his body erect and setting mindfulness to the
  fore [lit: the front of the chest]. Always mindful, he breathes in;
  mindful he breathes out.


The Buddha himself rarely made a clear distinction between Samatha states and Vipassana states, to him it was all meditation that co-existed, he  often called out the jhanas specifically.

A lot of the modern emphasis on Vipassana, and the resulting mindfulness movement is based on an excessive reliance on the Abhidhamma (itself a commentary), and the Visuddhimagga (which is brilliant, but has some controversial roots since Buddhagosa is said to have burned up ancient manuscripts that preceded it for reasons unknown).


  In the few instances where they do mention vipassana, they almost
  always pair it with samatha — not as two alternative methods, but as
  two qualities of mind that a person may "gain" or "be endowed with,"
  and that should be developed together. One simile, for instance (SN
  35.204), compares samatha and vipassana to a swift pair of messengers who enter the citadel of the body via the noble eightfold path and
  present their accurate report — Unbinding, or nibbana — to the
  consciousness acting as the citadel's commander. Another passage (AN
  10.71) recommends that anyone who wishes to put an end to mental defilement should — in addition to perfecting the principles of moral
  behavior and cultivating seclusion — be committed to samatha and
  endowed with vipassana. This last statement is unremarkable in itself,
  but the same discourse also gives the same advice to anyone who wants
  to master the jhanas: be committed to samatha and endowed with
  vipassana. This suggests that, in the eyes of those who assembled the
  Pali discourses, samatha, jhana, and vipassana were all part of a
  single path. Samatha and vipassana were used together to master jhana
  and then — based on jhana — were developed even further to give rise
  to the end of mental defilement and to bring release from suffering.
  This is a reading that finds support in other discourses as well.



Is it being suggested that you should be in the jhanas before going on any further in this meditation?


No, that isn't the intent.