This is a technique to clear out the side passageways of prana- the ida and the pingala. The basic technique involves breathing in through the left nostril and breathing out the right- then in through the right and out through the left. That completes one cycle.
When performing this technique it is often recommended to use the Vishnu mudra (index and middle finger bent) to close nostrils. The thumb pinches one nostril and the ring finger pinches the other.
The recommended ratio at least from Himalayan Institute is 1:4:2 – this is different than the ratio recommended by Aleister CrowleyAleister Crowley
in his libri for the A.'.A.'. (he recommends 1:2:4).