http://www.scootworks.com/rdrc/aerobatics/snap.html

Found it!

When you think about it - you're doing an instant spin along the direction of travel. Normally a spin is entered by slowing the plane and stalling it deliberately. Here, you're whipping it around while at speed.

That's why you see those cool videos on the Crash forum of the wing tubes ripped out of sheeted foam wings on the 33% planes. Just 'cause it can do it don't mean it can do it without risk.