It's just a highback that leans to the inside instead of straight up and down. It helps the highback align with the back of your boot, especially if you have a wide stance. Here's what Burton has to say about canted highbacks:
"Experience greater control and seamless response with our canted hi-backs. Found on the majority of the line, this anatomically accurate, ergonomic design mirrors the contours of your left and right legs for a more comfortable, natural connection."