Heavy Air Mainsail Trim
Tightening the backstay is standard proceedure when in heavy air because tightening the backstay does a couple things at once. When you tighten the backstay you tension the forestay. This flattens and depowers the jib while opening up the leech area up high. This moves the apparent center of effort forward and downward. Tightening the backtstay on a modern righ also induces a little mast bend which blades out the mainsail and opens the leech up high moving the mainsail apparent center of effort forward and downward. Combined this reduces heeling and weather helm.
I really question the purpose of applying the preventers when beating or reaching in a blow. Proper use of the vang and properly dropping the traveller in gusts really should be all that is necessary to stabilize the miansail. Applying the preventor really puts the boom and mast at risk should the end of boom dip in the water.