To the human leg, Dar; the horse yields to pressure or stimulus from the rider's inside leg.
However, as with most short-form language, you are not to take the term 'leg yield' literally. It's not as simple as you press-he moves.
There are older threads on the subject of leg-yielding in this Forum. Use the Google advanced search function as directed in the thread above this one, and try keywords 'leg yield', 'untracking', 'stepping under with inside hind leg'. Cheers -- Dr. Deb