When carving canyons I place my arse, body, shoulders and head in side the corner so only my thigh is on the seat. Keep your head square, or level so you retain the best balance, tilting your head is a no no for letting your ears do their balancing job. Putting just your shoulders or arse in really is not enough.
You are just using your body weight as a counterweight. For every bit of weight you can get inside the corner, you will need less degrees of lean, if that makes sense.
Since carving on the street can be sketchy, I would rather have too much body inside, and the ability to corner or brake hard, than not have enough, and something happen that left me shorthanded on Busa capability and no time to rearrange before sometime bad happened!
Maybe Kirk can go into this a bit better.