This means control of prana. People ordinarily think of prana as breath but that is not true. What is true is one of the function of prana is breathing
When shakti is awakened – eventually you experience the difference. After a Guru awakens shakti, if and only if he tells you to do the breathing exercises of pranayama you can hold the breath either within or outside or both. There are some secrets in this which are not written in books The Guru will tell you average where breath is held within or without
Without is called bahya Kumbhak. Within is called antarkumbhak. Besides this purak means taking breath within. Rechak is exhaling. There is also a word rechak purak and purak rechak. These things can be explained by your Guru
Once you have a Guru , he can tell you where and how to hold breath. Antarkumbhak or bahyakumbhak. Doing kumbhak is dangerous. It can cause health issues if not done properly. Yogananda was aware of it and suggested bahyakumbhak. Having said that, he did teach antarkumbhak also
They both have advantages and secrets. These things – the secrets are not revealed and not written in most texts. You have to go to a Guru. Some health complications can’t be reversed.
So although Yoga is the best,you need a Guru to be safe in the journey. He can tell you which kumbhak is best for you. It depends on personality, health amongst other things
I hope this helps