[dquack]

As much as I hate to admit it, I have sailed in some pretty gruesome storms. I have been aboard several times when the mast has hit the water.
One note about a boat being knocked down;
A wave 1/5 of the boats length can knock it down and even roll it. Hitting the water with the mast is usually not what breaks it, it is the boats ability to right itself with tremendous force that usually breaks the mast or tears sails. If your boat is ever knocked down by a breaking sea, just stay on deck and it will right itself (if it is a typical sailboat with a heavy keel). A tether that keeps you on deck is the answer here as well.
regarding a boat turning "Turtle". the sad fact is that most boats will stabilize upside down if conditions exist to get them there (breaking seas). Usually, however, they will also right themselves when the next wave comes along and tips the keel past the place where it is over the boat. The keel wants to be below the boat. The odds of the boat being turtled and staying there are very slim. The odds of a sailor slipping on a canted wet deck are much more probable. Preparing the boat so that you stay aboard is clearly most important. Having a way to unhook your tether from the harness end is important too, but very unlikely to be needed. The sad fact is that we are told of the importance of being able to unhook our harness under load because it allows us to get off the boat to begin with!

For some reason, rather than setting the boat up properly, we try to redesign the equipment to make up for installation flaws!?!?!
There is just too much bad information out there.

Don