The process of setting up a barber haul is very simple, all you need is a snatch block!
On a beam to broad reach, the headsail is eased, causing it to twist and loose efficiency.
The first step is to ease the headsail until there is slack in the line but not so much that the sail begins to flog. This will reduce the tension on the sheet, allowing you to place it into the snapshackle by hand. Then sheet the sail back in and trim to the apparent wind.
Now the headsail is able to hold a better sail shape, resulting in higher efficiency and more speed through the water.
With the better sail shape, you can reach your downwind mark in less time and more comfort!