What is a whip stall? I was reading 14 CFR 23.3, which discusses airplane categories. In normal and commuter categories, stalls (except whip stalls) are allowed. The regulation, however does not define "whip stalls." What is meant by this term?
The clearest definition I could find is from an AOPA quiz (of all places):


  The airplane is pitched up into a vertical or near-vertical climb. It
  then pauses, slips downward momentarily (tail first), and then whips
  into a steep, nose-down attitude.


The tail-first slip backwards seems to be the defining characteristic of a whip stall. Interestingly, Whip Stall and Tumble Awareness is included in the FAA's Powered Parachute and Weight Shift Control PTS, but they aren't defined there either.