Hi,
Just to follow up on Diablo's most succinct ;-) (and correct) summarization: Some snap criteria . . .
At the entry to the maneuver the aircraft MUST:
- Abruptly pitch positive or negative to briefly set the wings at a critical angle of attack to cause an immediate semi-stalled condition. In other words: no pitch departure, no points.
- Yaw to unbalance the airflow between the wings, and therefore reduce the critical angle of one wing while increasing the other.
- The initial pitch and yaw (can be made simultaneously, or as a quick one-two) must immediately produce rapid auto-rotation - note: the exceedingly light wing loading of scale aircraft necessitates aileron input combined with pitch and yaw to present the appearance auto-rotation.
- Throughout the snap-roll the aircraft must remain in auto-rotation by continued application of the initiating pitch, yaw, and roll control inputs.
- A translation to aileron rolling at any stage before the required degree of rotation has been completed would mean that the snap has ended early - a certain 1 point / 10° downgrade. - At the correct angle of rotation the roll should cease abruptly, and the aircraft should continue along an axis closely parallel to the extended pre-roll axis. Read as: some line displacement is NOT a downgrade - in fact, some displacement should be expected if the snap is properly executed.
- Check carefully that the last part of the auto-rotation is not turned into an aileron roll to assist accurate end-stop positioning, a commonly adopted ploy that must be penalized. An extreme but VERY common example of this: 1 1/2 snap is presented as 1 snap + 1/2 roll, which = 0 points.


Whether the aircraft is flying fully "on the wing" or not is irrelevent with regard to what must occur in a snap. The fundamental aspect to a snap is autorotation generated by a differential in lift between the two wings - regardless of flight path - up, down, horizontal or otherwise - (your in trouble if its otherwise). That lift differential is achieved by a rapid change in the wing's angle of attack combined with, or immediately followed by, yaw. As noted above, scale models generally also require the application of aileron due to the very light wing loading.
Hope this helps.

Ty

sp. ZDZ Engines, RC Showcase