Gaja in Sanskrit means elephant; karni is an action. An elephant sucks water and then raising the trunk, throws the water out like a jet. In a similar fashion, a student of hatha yoga can pass water out through the mouth in spurts, which appears like the act of the elephant. Gajakarni is a yogic technique of throwing out the contents of the stomach.
Gajakarni is described in Hatha Yoga Pradipika in the following words – it is an act of vomiting out the contents of the stomach, brought about by drawing up the apana to the throat. With practice, the nadis chakras, i.e. the centers of the channels, are brought under control and then one can do the act like an elephant.” (II.38)
Thus Gaja Karani may be called a technique of controlled vomiting. It involves a control of the vomiting reflex. Usually that reflex causes an act of vomiting, which is sudden and rather vigorous.
The stomach is emptied by a sudden contraction of its wall under an impulse which starts in the throat. The impulse is generated willfully after drinking three or four glasses of water. The water comes out in a controlled manner like a jet. With practice, the student can perform the act very naturally, which has a considerable therapeutic value as a stomach wash.
Although the author of Hatha Yoga Pradipika has not included this technique among the shuddhikriyas (cleansing procedures), he has described it just after describing them. It could very well be called a variety of dhauti (cleansing); more specifically vamana dhauti. Vamana means vomiting.
Source:
Hathapradipika of Swatmarama (1970) – Swami Digambarji and R G Kokaje (eds) – Kaivalyadhama – Lonavla
Encyclopedia of Hinduism Volume III page 169 – 171. - IHRF