Superman double backflip
Impressive stuff, which is obviously due to the law of conservation of angular momentum – of course!
The trick begins with a tight backwards flip, which happens quickly because all of the weight (human + bike) is distributed close to the center of gravity. By opening his body up in the middle part of the flip, Roberts slows down the rotation, just like a figure skater, diver, or ballerina would by throwing out their arms or legs. And then he gathers himself back onto the bike, which spins quickly again because the weight is all back close to the center. Boom, physics.
Via Kottke.org