What is a whip stall?
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.