This glitch is a small time-saver in Mario Kart 64. After coming out of the tunnel, look for where the gray meets the brick wall (on the left). On the sixth brick from the left (behind a tree), use a mushroom, and jump as you hit the wall. You fly over it, and cut a few seconds off of your time.
Why it works Edit
Possibly an area where the two wall sections weren't spliced together correctly, and jumping at the correct time and angle causes you to go through the seam.
This glitch was discovered by me.