The Impossible is a trick where you pop the board off the tail, move your front finger out of the way, and let the board wrap around your back finger, catch it, land, and roll away. One of the best ways to learn Impossible is just to take your hand and pop the board as hard as you can and get it to spin around your finger like that. That's the first step. Next, once you can get the board to spin around your finger, you want to bring your front finger back, catch it, land, and roll away. A difficulty many people have with learning this trick is getting the board to spin around their back finger. I think finger positioning plays a big role in whether or not the board will spin around your back finger. If you position your back finger in a way where it's hanging off just enough, that will help you get under the board a lot more, allowing you to scoop it. Now, during the trick, after you first pop the board, the first thing you do is bring your back finger up and out of the way, allowing it to spin around your back finger. The way to do that is really, as soon as you pop it, just peel your hand back, but not too far because you want to keep the board rotating around your back finger. Also if you put your front finger hanging off a little bit, that helps you get it out of the way during the trick. And that's how you do the Impossible on a fingerboard.