I don't know what its called in English, but its when you have the ball and lets say your a lefty, so you cross the left leg behind the right leg and strike the ball with the left foot, i've seen it done when a lefty wanted to center from the right.
Yeah, tricks are really useful. I'd say 20% of the ones done are useless (I've found myself spinning with both feet on the top of the ball?!). The rabona can be useful too. It's not all for flash. Let's say the ball is moving quicky from your right to your left. If the ball gets too far for you to strike it while balanced with your left, then you do a rabona. The rabona is also useful in tight spaces when you must strike the ball is it lies. It also works when you want to turn and shoot in the same motion. ------------------------------------------------- The best trick however is the meg and the hundred variations that can be done.
Scored a nice goal when a teammate's shot rebounded off the bar to me at the spot. It was too high and too far off to my right to properly volley it, so I flicked my right foot up behind me (I was facing the goal) and the outside of my foot, behind my head and slightly off to the right, smacked the ball into the upper left corner. Pity that it was during pregame warm-ups.