The science of Haṭhayoga prescribes ‘dhauti’ or cleansing of the body to make it pure, healthy and strong so that it can withstand the rigours of Rājayoga. This is one of the several techniques and is of four kinds: antardhauti, dantadhauti, hṛd-dhauti and mūlaśodhana.
Antardhauti, the first of these, is again of four kinds: vātasāra, vārisāra, vahnisāra or agnisāra and bahiṣkṛti.
In vātasāra, the purification is effec-ted by inhaling air (vāta = air) by shaping the lips like the beak of a crow, filling up the stomach with it and expelling it through eructation.
In vārisāra, warm saline water (vāri = water) is drunk and expelled through the rectum. This is later on followed by vamana-dhauti, drinking saline water and vomitting it.
In vahnisāra, navel knot is pressed against the spinal cord by exhalation and contraction of the muscles of the stomach.
Bahiṣkṛti is similar to vātasāra except that the air is expelled through the rectum instead of through the mouth.
These techniques not only purify the body but also free it from diseases. However, they should be practised only under the personal guidance of an expert haṭhayogi.
See Gheraṇḍa Saṁhitā 1.14-22 for details.