What you are expierencing is what is known as an accellerated stall. By sharply feeding full down elevator you cause the wing to exceed the "Critical Angle of Attack". Remeber that a stall has absolutely nothing to do with airspeed, only angle of attack. The reason it snaps after the stall can be caused by a number of things. Possibly engine torque (think about a power on stall, the plane is more adept to break the way that the engine torque is pulling it. A slightly warped wing (caused the crash of my Global Raven...slightly warped wing....a little too much elevator on approach.....BAM!!!). some airplanes just have a tendancy to fall one way or the other in a stall (as opposed to just the nose falling straight forward). Thats about the extent of my aerodynamics knowledge....maybe my cousin will read this and prove me wrong....(Aerospace Engineer, Embry Riddle).

Hope I made sense (Yeah right!!! )

Micah (SloFlight)