I'll give it a shot
I thought that others with a better sailing vocabulary would jump on this, but I'll give it a shot.Barber haulers are great for controlling the angle of pull on a headsail sheet. Mine is a basic block and tackle (similar to your main sheet, but the loads are much less, so fewer pulleys are needed) hooked down to the toe rail and up to a snatch block that snaps around the genoa sheet.Depending on where you attach it, it can either pull the sheet down or in. For example if the top of the head sail is luffing before the bottom, put the barber hauler on and pull the sheet down a bit until the whole leading edge of the sail luffs at the same time.If you are using a smaller headsail than normal, it may want to be sheeted in closer to the centerline of the boat for best performance. Here, a barberhauler could be attached near the mast to pull the sheet more towards the centerline.For many new fangled fancy boats, barber haulers are not needed because these boats have multiple and adjustable genoa lead tracks that allow one to get the perfect sheet angle regardless of conditions.