He did it with special shoes that quickly slid into pegs that rise out of the floor at just the right moment. Also helping the effect were rigid anklets that worked like ski boots, supporting Jackson and his entourage of dancers as they leaned forward at that magic angle.
Cool, but you know what I want? Moonwalk shoes. One time I went to a club with a friend of mine who can moonwalk. He only did it for like five seconds. Thirty women got pregnant.
Hit the jump for a video of Michael performing the stunt, action is at 3:50.
Thanks to FDSY, rough criminal.