When sailing downwind, you will ease your headsail sheets to let them catch more wind. The sail will tend to twist when eased too far, losing efficiency and costing you speed through the water. This is where the Barber Hauler comes into play, controling twist while keeping the headsail eased.
Barber haulers are simply sheet leads that are set closer to the beam of the hull than your regular jib sheet tracks. This allows you to sheet the sail to the toe rail rather than far inboard.
If you are sailing downwind in light to moderate airs, attaching a snatch block to the toe rail close to where the clew of the sail is will do wonders. This gives the sail a strong downward pull which will eliminate twist from the sail while putting less tension on the foot of the sail. The end result is a very full sail with minimal twist that will fill with air and send you rocketing downwind.
The reason snatch blocks are ideal for this set up is they can easily be removed from the sheet should you change to an upwind course. Removing the barber hauler is as simple as opening the snatch block and removing the sheet. If winds are very light, this can be done with the sheet under load (as long as you can safely hold the sheet under tension). If the winds are stronger, it is idea to jibe onto the opposite tack and remove the barber hauler now that the sheet has become the lazy sheet.
Next time you are sailing downwind, consider using a barber hauler to control headsail twist and enjoy the added speed through the water while on your run.