Not understanding Jhana Mastery Of Reflection? What is the goal for this mastery exactly? I'm guessing it's to look back at your jhana meditation and see what happen. What went wrong, what are things you can improve, what lead to jhana &amp; lsot the jhana etc.

Is this correct? I want to make sure i understand completely.
Pa-auk monastery tradition:

If you ask this question at pa-auk, they will recommend "you should in meditation, first". Because mind and mind factors arising are very fast and complex. So, it is not possible for no-attained-jhāna-person to clearly understand jhāna factors. And I am agree with that recommend, too.

This image captured from TEACHING &amp; TRAINING book of Pa-auk monastery:



From path of purification pāli, pathavīkasiṇaniddesa:


  ayaṃ panettha atthappakāsanā, paṭhamajjhānato vuṭṭhāya paṭhamaṃ vitakkaṃ āvajjayato bhavaṅgaṃ upacchinditvā uppannāvajjanānantaraṃ vitakkārammaṇāneva cattāri pañca vā javanāni javantiฯ tato dve bhavaṅgāni, tato puna vicārārammaṇaṃ āvajjanaṃ, vuttanayāneva javanānīti evaṃ pañcasu jhānaṅgesu yadā nirantaraṃ cittaṃ pesetuṃ sakkoti, athassa āvajjanavasī siddhā hotiฯ


Translation:


Emerges 1st jhāna, then...
Bhavaṅga-minds auto arising, break it, then ...
Thinking of vitakka by only 4-5 javana-minds arising, then...
Agree only 2 bhavaṅga-minds arising, break it, then...
Thinking of vicāra by only 4-5 javana-minds arising, then...
Loop 4-5 steps by 5 jhāna-elements' sequence, until you will attain Jhana Mastery Of Reflection.


How to notice the Jhana Mastery Of Reflection attainment?

General person use 7 javana-minds arising   per thought and uncountable bhavaṅga arising between thought, but the jhāna-master must use lesser javana-minds and lesser bhavaṅga-minds to take mastery jhāna.

The practitioner attained right, exact  Jhana Mastery Of Reflection, when he can meditate  Jhana Mastery Of Reflection follow above step without any getting struck, such as more than 2 bhavaṅga-minds arising or more than 4-5 javana-minds arising.

Still doubt?

There are many jhāna masters at Pa-Auk Forest Monastery. This is better way, because below quote was written in the path of purification page 94:


  So if someone with cankers destroyed is available, that is good. If
  not, then one should take it from a non-returner, a once-returner, a
  stream-enterer, an ordinary man who has obtained jhána, one who knows
  three Piþakas, one who knows two Piþakas, one who knows one Piþaka, in
  descending order [according as available]. If not even one who knows
  one Piþaka is available, then it should be taken from one who is
  familiar with one Collection together with its commentary and one who
  is himself conscientious. For a teacher such as this, who knows the
  texts, guards the heritage, and protects the tradition, will follow
  the teachers’ opinion rather than his own. Hence the Ancient Elders
  said three times, “One who is conscientious will guard it.”


For alternative translation see: 

https://buddhism.stackexchange.com/a/23977/10100

For word definition:


http://www.palikanon.com/english/sangaha/chapter_1.htm
http://www.palikanon.com/english/sangaha/chapter_2.htm
http://www.palikanon.com/english/sangaha/chapter_3.htm
http://www.palikanon.com/english/sangaha/chapter_4.htm
https://www.accesstoinsight.org/lib/authors/nanamoli/PathofPurification2011.pdf