Any time you get weightless, as with severe turbulence, you should have one leg out of the harness. If you kick the keel with both feet together (think "two-fisted hammer-punch"), you could break the keel. If you spread your feet, then one foot hits the sail, on each side of the keel, and chances are good that the keel will not break. Your feet should spread automatically, any time you go weightless, as a reflex response to weightlessness. The most common cause of weightlessness is getting dumped out of a very strong thermal.