To help the body to clear away toxics.

Start by sitting comfortably with you back straight.
Take a deep breath. Hold the breath for 1sec, while lifting you chin a little.
Slowly start breathing out, by holding back the breathe with tight lips so that you have to use your stomach muscles to push the air out.
While breathing out imagine how the impure air starts withdrawing from the lower body towards your mouth, till you have inhaled it all out.
Breathe out all air from lungs, till your body naturally starts to bend down a little and you chin lowers towards you chest.
Hold again for 1sec, before taking another deep breath.
If you wish you can imagine how you breathe in light or pure air.

Repeat only three times in one session! This is a very powerful practice if done properly.