Blood flow directly impacts hair growth as blood delivers nutrients and oxygen to scalp and hair follicles which helps grow healthy hair. That is why most hair masks and treatments are aimed at improving blood circulation in the scalp.
The leech therapy is excellent at improving blood circulation thanks to its blood thinning and blood cleaning properties (see here), it helps the hair follicles to receive more nourishment so the hair grows faster. The leeches will be applied to balding areas. Usually, up to 3 sessions are needed to activate the hair growth.