When you shoot out of the barrel cannon, you will come a to a U-turn. You will then realise that both sides of the track have the ramp to do tricks. Go up the ramp on the INSIDE of the bend, not too much though. Then your character will starting going around the bend very quickly.

Trust me, I got a box, then done the bend. ended up from 5th right to 1st, with triple red shells (and also owned Chrono on that race too XD).