The roastbeef is a type of skateboarding/snowboarding grab that is relatively advanced, and can leave you looking mighty stupid if you don't do it right.
To do a roast beef grab, you can use a half-pipe, quarter-pipe, kicker, launch, or other aid to get plenty of air. When in the air, you simply reach toward the heelside of your board with your leading hand, and grab it. Simple.
"But wait," you may say. "Isn't that just a melon grab? What's so special about that?" Well, I did neglect one minor detail. When you reach for the edge of your board, instead of dropping your hand behind your back, you reach between your legs! Isn't that exciting? Then tweak it a little, and everything looks cool.
The risky part is, dropping your hand between your legs can be a risky proposition, especially when you are on a skateboard. This is because you can knock your skateboard away with a poorly timed grab. Also, taking your hand out from between your legs must be done smoothly. Failing to do so may leave you with a fairly mangled paw that resembles, well, the sandwich meat this grab is named for.