What happens to my body when I breathe out? It is suggested, that one concentrates on the breath while meditating. It is also suggested that one breathes out and remains in this state for a moment, to feel the emptiness. What happens to my body while I remain in this breathless state?

Does it cause relaxation and if so, why?

I'd like to know something about the physiological effects of this aspect of meditation...
There are a few mental cultivation techniques which involve breathing. E.g. there is Paranayama which pre-dates Buddhism. Here you do try to regulate the breath.

In Anapana there is no regulation of the breath. Hence you do not breath out and try to rain in that state. (It is also suggested that one breathes out and remains in this state for a moment, to feel the emptiness.) During the natural cause of calming the breath (bodily fabrications) your breath becomes calm and stops. You should not force it through actively focusing on the breath (without your mind wavering to other subjects) helps towards this.

Breathing does have feeling associated feelings tied to the body.


expansion and contraction
oxygenating feeling / pertaining the corporal structure - when you are a reasonable level of tranquillity with all other bodily feelings subsiding, the bodily feeling in the in breath is slightly different from the feeling you have on the out breath. / 4 elements.
contact / touch - feelings and / or 4 elements


In addition your breath becomes shallow and stops after a while as you go deeper into meditation. This happens when you calm the bodily fabrications (relaxing the bodily function) as the breath is the body conditioner. Relaxation of your breath through focus on the breath and breath alone relaxes your body and vice versa. The way to calm the body is to look at all bodily sensation scanning from head to the toes and back again with no reaction to the sensations. This progressively changes the sensation until you get Piti followed by Passadhi followed by Samadhi (The Upanisā Sutta, The Das’uttara Sutta, etc.). This also calms you breath.

Physiological is that breath meditation:


calms your breath if you practice correctly 
calms your body and releases tension if you practice correctly
makes your bodily feelings more pleasant and tranquil