Wow. It's hard to say whether this skill is a talent or a curse. Either way, the two contestants in this attempt at the world record for most head-spins in sixty seconds must have seriously strong stomachs.
At first they speed up their spins by using their hands to turn themselves faster, but there must be a limit on this, since they don't use the floor again once they slow down towards the end. And while the second contestant absolutely slaughters the first, nearly doubling the number of spins, their techniques are fairly similar.
Like skaters spinning in place on the ice, they both take advantage of rotational dynamics. They start with their legs and arms out wide. During this phase they have a high momentum of inertia, meaning that each contestant's mass is distributed widely about his axis of rotation. When they bring their arms and legs in tight, transforming themselves into spinning tops, their moment of inertia decreases. This causes them to accelerate, cramming in more complete spins per second, because their angular momentum, which is a function of angular rotation and moment of inertia, wants to remain constant.
So, if the moment of inertia drops, the angular rotation increases, and the head-spinners edge closer to the record. Please try this at home, and let us know how it goes. A video diary would be preferable.