I have witnessed many a spin that was described as a knife edge spin that frankly wasn't. Most often a version of a flat spin entered from a snap is described as a knife edge spin because the plane rotates through knife edge getting there. This is particularly true when the plane is not completely stalled and the entry (snap roll) gyrates the plane a bit.

The information I have posted above is direct from the video "Wringing it Out" by Dave Patrick of TOC fame and now of his own company. He mentions that the Ultimate rolls out of the knife edge spin for some reason. Dave is the designer of the Carl Goldberg Ultimate as well as the ones his company produces.
View the tapes and see what a true knife edge spin looks like.

I'm sure there are cases where a plane will knife edge spin with different inputs however the described method is all but a fool proof way.

Fly and enjoy,
Bill