After pranayam, breath regulation is pratyahar, abstraction. Pratyahar is returning back, coming back -- coming in, turning inwards, returning home. Pratyahar works as a bridge between the four, the yoga of the outside, and the last three, the yoga of the inside. The process of return is when the energy is no longer moving outwardly. It is turning inwards. Ordinarily, the energy is moving outward. We waste so much of precious energy in useless things like gossip, worry, random thoughts etc. Now in pratyahar the energy is to be conserved and build up as we will need it in the last 3 limbs. Energy is sapped because we have given the senses too much power over mind. Do not waste the precious energy in senseless situations at any cost, as in the end you will regret it.
This has to be deeply understood: that senses by themselves are impotent -- unless you cooperate. That's the whole art of yoga. If you don't cooperate senses close. If you don't cooperate conversion starts. If you don't cooperate pratyahar comes in. That's what people who are sitting silently for hours, for years, are doing -- they are trying to drop the cooperation between themselves and their senses. When the energy is not obsessed to see, to hear, to touch; the energy starts moving inwards. That is pratyahar: movement towards the source, movement towards the place from where you have come, movement to the center. Now you are no longer moving to the periphery.