Ao does something with his right leg, but doesn’t step where his leg went

Far as I can tell (really wish the sumo association would get on board with having multiple angles of the match--they've got like 2 but it should really be 4 or 5) he transfers his weight as much to his right side as possible to meet Koto's incoming thrust on his left shoulder so that doesn't shove him out. His right leg forward movement is part of that weight transfer, and then he pivots as fast as possible off his right foot once it lands to gain better ring real estate and goes to finish the match from there.