BEagle seems to have it right in my opinion though the use of the term "hammerhead" might confuse those from across the ocean where it describes what we in the UK know as a stall turn.
My own definition of a "whip stall" is one where the aircraft is placed in an extreme climbing attitude, say over 70 degrees nose up, and at the stall the aircraft very smartly swops ends in pitch into a vertical dive. If the climb attitude is just below 90 degrees then any tailslide will pitch canopy up and if just over 90 degrees then any tailslide will pitch canopy down. Such a manouever certainly falls into the aerobatic category and if carelessly executed in terms of a very firm grip on all controls can certainly result in structural damage.
Cheers,
Trapper 69