When the three bandhas (jalandhara, uddiyana and moola bandha) are well perfected the rechaka and pooraka (inhalation and exhalation) do not function, the flow of senses gets purified and kevala kumbhaka takes place. Therefore experience the vidya (pure knowledge) in the form of kevala kumbhaka.
One should practise the above while fixing the mind in the anahata chakra with total awareness, until at last the breath and the mind become motionless. It is then and there that the spontaneous kumbhaka known as kevala kumbhaka is experienced.
There are innumerable varieties of kumbhakas described in hatha yogic tradition, but none can equal the kevala kumbhaka, because in no other kumbhaka does the prana become steady.
Adishankaracharya, Yogataravali (8-10)