The rider being passed must brake because his line is now blocked. Brake check - Going into a turn, a rider will suddenly brake hard, forcing the often startled rider directly behind him do the same thing. This brake check allows the rider in front to gain a bit of distance while the following rider recovers. The rider must return his leg to the normal riding position in time for the landing. Casing it - Coming up short on a double or triple jump and landing on the top of the last jump instead of clearing it.
Casing refers to landing on the frame rails and engine cases.
He cased it hard over that triple. Clicker - A stylish trick a rider does while airborne. He lays the motorcycle flat while bringing the back of the bike around. Triple jumps are the signature obstacle of Supercross, requiring cool nerves and precise throttle control. DQ'ed - Disqualified.