Heres another thread on this same topic. although you basically use the rim of your drum as a fulcrum. the rocking motion gives you a hit on the way down and a hit on the way up. but heres 2 pages already discussing everything you might ever want to know about it.
P.S. its called the free hand technique. gravity blast is the name the metal guys gave it in context of a blast beat.
http://drumsmylife.com/-quotgravity-blasts-quot-vt4915.html
