Around the fourth turn on this track, you should notice a brick wall on the right. This shortcut only works if you have a mushroom. First of all, before you turn, go up the hill a little bit. Then drive toward the wall. Just before you go onto the road, hit Z. And just before you fly off the road, jump. If you timed it all right, and you drove to the right spot towards the wall, you should make the jump over the wall and shave a few seconds off of your time! Trust me, this takes time and practice, it's tuff!