You are talking about a snap roll sounds like to me. You still do the same thing from knife edge its just that the elevator is acting as the rudder since its vertical. Pulse the rudder to momentarily increase attitude and then full up elevator, and full rudder/aileron simultaneously. Get out of the elevator (just as you would the rudder in an upright positive snap) a little early and it should come to rest back on knife edge if your timimgs right.