Last fall's install of my Fisher Canvas cover on my boat.
The 2x4, lag screwed across the ends of the 2x4 main supports is recommended by Fisher to prevent the timbers from twisting 90 degrees and thereby laying "flat" and not "on edge" for max support. Also per Fisher "standards" there is a brass grommet lined hole at the end of each support pocket for a lag screw and flat washer which prevents the supports from sliding out during a high wind storm. High winds cause even the heavy Fishers to flap, thereby shaking out the main supports. Boat is in yard @ the Lavallette YC and sees quite strong winds over the winter.