This is a breakdance move that is significantly more difficult than the closely related handspin. For the basic position information, check the write-up on the handspin.
Now, once you've got the handspin position, you need to be able to hop. It seems impossible at first, but it can in fact be done. Then, combine the spinning and hopping into one awe-inspiring movement.
Though harder to learn, this move has some practical benefits to the handspin; mainly that you no longer rely on your skin somehow electing to stay attached to your hand rather than the nice sticky floor it's in contact with as you spin around. Also, it is possible to continue rotation much longer without pushing off with the free hand because you can use the support arm to turn yourself along like a wind-up toy.
Of course the clearest benefit of this move is that it looks way more impressive than a handspin (which is saying a lot). It looks impossible, like you're going to fly away like a helicopter. Very cool.