After lecturing people on this forum on how to manage the jib in strong winds, I went out today almost put the boat over because I didn't follow my own advice. One more time:
With sails trimmed in strong winds, when the boat takes a knockdown strong enough to lose control (rudder and centerboard too far out of the water to steer the boat):
* Both sails have to be slacked.
* Slacking the main alone will not stop the boat, it will allow the jib to turn the boat downwind.
* Once the boom hits the water, the jib has enough power to take the boat the rest of the way over.
The rule of thumb I ignored was keeping the jib sheet in my hand whenever the boat heels continuously over 15 degrees. Today I was handling the main alone, and when the gust hit, the jib just fell away. Once the control surfaces were out of the water, the boat couldn't round up on its own. I released the main, but it could not swing out any farther than where the boom hit the water. I was climbing over the high side as the still-trimmed jib took us down. I don't know why the boat didn't capsize; I think the gust just ended. The leeward coaming miraculously resurfaced and all I suffered was a few gallons of water in the cockpit.
By the way, conditions were breezy but hardly extreme. I measured the wind at 12-16 knots, and I would guess my knockdown gust was maybe 18 knots. Furthermore, the boat was rigged very conservatively. I had double-reefed the main, released the traveler, and tensioned the vang. I had done everything I could except sail the boat properly. Or douse the jib altogether, which I did until the adrenaline had metabolized out of my bloodstream.