Onehanded, Continuous Tumbles - The hat keeps tumbling, in your hand, continuously, infinitely, forever, or at least until you stop.
This trick is an extension of the Tumble - so go learn that one now.
You'll notice that after you have performed a tumble, you are holding the hat the wrong way round to perform a second tumble immediately after it. This is where the extension comes in.
During the first tumble (which is a standard tumble), the hat rotates 1.5 times. The second tumble is similar, but the hat is thrown without a change in grip, it still does 1.5 spins, which means that it is now facing the same way as when you started!
Inorder to give yourself space for the second tumble, it feels like you need to throw it a tiny bit higher or faster.
Try to make it all flow together into a continuous movement, the hat shouldn't ever pause, the hand acts more as a guide to keep it rotating in the right direction.
A nice variation on this is to tumble the hat infront of you, switching hands between tumbles.