My understanding of the Slingsby accident is that it was not the "nose nod" itself that caused the problem but the fact they were picking up the wing with rudder and in fact the oscillatory bit they were taking about was the rudder rather than the "nodding", unfortunately for all the detailed analysis the AIB do not explain enough WIHIH in this. Personally I like a student to fully recognise the full stall themself before recovery which should happen before the first actual "nod" and they certainly should not be picking the wing up. Normally what happens is students call "stalled" and recover well before the actual stall itself.