More important than "flattening" the main is to hold the boom down as you ease the sheets
(while sailing off the wind).
Without a boom vang
the boom lifts up and the mainsail
twists off and spills the wind
. Close hauled the mainsheet pulls mostly downward and prevents the boom from rising but when the sail is let out the pull of the main sheet is no longer mostly downward, and the boom can rise. On your Bayfield with the mainsheet at the transom this is exaggerated since letting the sail out only a small amount changes the angle of the mainsheet.
The comments about a possible "Chinese gybe" where the boom lifts high enough to strike the back stay and the benefits of eliminating the topping lift
Sailing is a sport, an athletic activity, not a sedentary one.
Fred Roswold-Fred & Judy, SV Wings, Mexico