The 50-50 has another application, as far as skateboarding goes. It is a position that was often used in freestyle skating, back in the 80's. Rodney Mullen can do these with the same ease that he does Caspers, and can even do flip tricks in between the two stalls.
The 50-50 stance is similar to a casper in that you are balanced on just the tip of the tail. The 50-50 is a much more upright position though, and involves the trucks. To get into a 50-50, the easiest way is to snap down one end of your skateboard, and jump into the air. Yes, like an ollie, only instead of sliding the other foot up the griptape, you let it flip over end for end. The board can also be guided by hand at this point. While in the air, move the board until it is nearly upright, standing on its tail. Now, plant your opposite foot (not the one that snapped the tail down) onto the truck that is on the bottom, like it is the rung of a ladder. In order to keep the board from slamming down immediately from this force, put your other foot on the griptape side of the board, supporting it in the same sense as a casper. There you are. Now, do some kind of crazy flip trick out, and you're a superstar for reintroducing an old trick to skateboarding.
If you want to see one done, I would recommend watching Rodney Mullen's part in Rubbish Heap, a movie put out by World Industries. He does a spin into a 50-50, and stays balanced in the position while spinning one and a half times. Frontside and backside.