When sailing dead downwind in heavy air on a boat with a symmetrical spinnaker, control of the boat eventually will become a problem. One wrong wave, an unexpected puff or momentary loss of concentration by the helmsperson can lead to a gybe broach. This video will give you tips on how to avoid a dangerous, boat breaking gybe broach.
To start with, you can’t sail the boat the same way in heavy air as you do in more moderate conditions. You have to trim the sails tighter and sail as much as 10-20 degrees higher than dead downwind.
To reduce the chances of a wipe out, you need to prevent the boat from heeling to windward. When the boat heels to windward, the center of effort shifts to windward, forcing to boat to turn to leeward. At high speeds this can make the boat uncontrollable because an attempt to correct the course results in the rudder stalling. This video gives some tips to help prevent gybe broaches.