This is shortcut in Mario Kart 64. To do this one, get the blue shell in Luigi Raceway. Make sure you are in first place as you approach the final lap. Right as you go to that lap, fire the shell, then QUICKLY turn to the left. If it works right, you will be knocked over the wall and into the tunnel. You have skipped half the course! Congrats!
Why it Works Edit
The blue shell knocks you over the wall and into the tunnel. Like I already said.
This glitch was discovered by David Wonn.