Anti-whip is the opposite of a whip. A whip(and anti-whip) is performed on a sled or motorcycle when airborne after hitting a jump. A whip is when you throw the rear of a sled/bike to the side and then bring it back straight for landing. Performing a super dirty whip involves bringing the machine perpendicular to the direction of travel and then back straight. Many times when hitting natural jumps, you must perform a whip mid-air to land on the transition landing properly. Anti-whip is performed when hitting natural terrain and whipping the opposite direction than needed to land straight, this is never intentional - sometimes the machine gets a mind of its own. At this point the rider usually suffers a hard landing and sometimes even results in wadding it up.
Dude 1: I was up at turnagain yesterday hitting the world record jump and wadded up pretty bad bro.
Dude 2: Dang brah, what happened?
Dude 1: My sled started drifting left when I needed it to go right to make tranny... It suck bro.
Dude 2: Anti whip!!!! I hate that, brah!!