Supastarrio | Mario Kart DS: Rainbow Road Glitch @Supastarrio | Uploaded 13 years ago | Updated 2 hours ago
Instructions:
1: Spam the R button while you're upside down
2: Keeping spamming it until your engine sounds like it's at 300cc
3: Then try to clip through the wall

It's not exactly easy to do, but this does work.