This is a glitch in Yoshi's Island DS (It might work in the original too, not sure). To perform it, go to an area with a chomp rock that you push uphill, and a way to take damage (The firebar part in Extra 1 works well). Now, as it gets to the top, get hit by the firebar/other enemy. Right when this happens, duck. The Chomp rock will roll downhill, and go right through you! However, I have been unable to replicate this glitch.
This glitch was discovered by Uberninja1337