Bill:

That is very interesting. I had assumed that the cabin top strap was for the 70% jib sheet in a real blow. Ordinarily the sheet would run through the block on the forward track to the jib clew. But in a blow, I would run the jib sheet from the strap, through a block on the clew and then back to the block on the forward track to the winch. That way the purchase on the jib sheet is doubled.

Another theory shot to blazes.