I think you'll find many uni sailors in agreement that the main is what finally puts the boat over, even down wind.
Speaking for myself, when a hard puff hits under spin I naturally drive down but hold the spinnaker for as long as possible. That can be difficult as the sterns start to lift. As a last resort, once it's clear the boat won't recover I dump the spin and make a stab at releasing the main. When I'm successful, the boat recovers nicely then it's just a matter of reversing the sheeting order and off you go. The total time is rarely more than 3-5 seconds.
The point is: that flat head sail, high up is the culprit. Imo, uni sailors would be better with something more like a pin head.
Interesting about releasing the main sheet, I always have been told the minute you release the main in an impending nose dive caused by the spinnaker powering up too much, it presents the top of the main more to the wind now coming from the rear, which simply makes matters worse as well as possibly putting even further stress on the mast which is now unsupported by the main sheet.
The really good sailors seem to never get into the position of the spinny over powering the boat and seem to be able to just go faster and faster all whilst on one hull. I remember John Pearce at a very windy second F16 champs I attended saying its better to get the windward hull even higher off the water in big breeze as it presents the side of the hull to the water and therefore has a better chance of not diving, as he won the event I have to probably believe him.
