I would think that you would need at least a little forward motion to enter a stable knife edge instantly. You just may be stalled out dropping. I don't think you can enter a knife edge from just any manuever. At least not instantly. Like someone else said, a snap to a knife edge is very easy. Or hovering and slowly transition into a knife edge is possible. But a knife edge spin has no forward motion that I know of so I can't see this being to easy. If you stop the knife edge spin, get it sideways and give it full rudder, even though it may drop for a while, with enough rudder authority, power, and alititude, it will level out. Kind of the like the down line of a knife edge loop.