Minifig rotatable legs on a stick - what is this?
The part in question shows up as a "Minifig Gravity Stunt Handle" (part number x817) on Peeron and a "Turntable Spinning with Sports Trick Handle" (item number bb128) on BrickLink. Looks like it appears in two sets:


Snowboard Super Pipe:


Skateboard Vert Park Challenge: