Breath-Infusion / Willpower Control
Apart from causing the achievement of the fifth stage of yoga, which is sensual energy withdrawal, Srila Yogeshwarananda is of the opinion that breath infusion causes energy willpower response in the psyche, something which is necessary if the yogi is to become a siddha here or hereafter.
What is energy power response?
That is when merely by his will power the yogi can cause changes in the energy content of the subtle body, where he can move energy around or exchange energy merely by a will power command, where he can transit to higher dimension just by wishing for it.
This is an important achievement. It is not the same as visualization where one mentally and/or emotionally works energy to produce a desired effect. This is where when the will power operates, it is effective just as when one wills the physical body to run, it may run and when one wills it to fly in the sky, it ignores the command.
Initially if a yogi wills that the stale subtle energy in the astral body be exchanged for fresh energy, nothing of the sorts will occur. But if the yogi does a breath-infusion method, that may happen. His command was the breath infusion practice which resulted in the desired exchange of energy. He used an indirect method because the energy in the psyche was unresponsive and very indifferent to his will power.
However according to Srila Yogeshwarananda, over time of doing breath infusion and practicing the compression and spreading of the infused energy, the yogi gradually attains willpower control. At least after being evicted from the physical body for good, the yogi may discover that the energy in the subtle body is responsive as desired.