One of the boom blocks is a ratchet block. The ratchet block works similar to your jib sheet winch to ease strain from the sheet. Particularly helpful in puffy wind conditions when you don't want to cleat the sheet and risk a knock-down. With the aid of the ratchet block, once you trim the main you can relax your grip on the uncleated sheet without easing the main. When you have to dump the main in a gust, just ease the sheet and, just like easing the jib sheet when around a winch, the mainsheet will slip right through the block. In light air, the ratchet mechanism automatically disengages and works just like a regular block.

Used in conjunction with your existing transom mounted fiddle block.