Archived Kimimaru4000 video from YouTube. Original description mostly intact:

This is a glitch in Paper Mario The Thousand Year Door that allows you to walk backwards. I found this glitch out by myself.

To do: Press and hold R to flip sideways. Jump and let go of R. Right before you land, jump again and do a ground pound. If done correctly, you should be flipping back to normal while jumping and doing a ground pound. Immediately after the ground pound, tilt the control stick in the opposite direction of the one you're facing.

This works in both directions, and it can work anywhere. It doesn't work if you don't ground pound.

Enjoy!

Check out my profile for more cool videos!