The traveler is a track that runs across the boat, athwartships, under the boom. It compensates for twist in the mainsail by angling the boom off of the centerline without allowing the boom to lift. This counteracts the difference in wind velocity between the top of the sail, which is generally in higher wind, and the bottom, in less wind, by increasing the efficiency of the bottom when the traveler is brought to windward. The traveler can also reduce heeling by raising the sheet attachment point, where much heeling force is applied, off center to windward. And it helps to de-power the main by spilling air when the sheet attachment point is lowered to leeward.
If the traveler attaches to the middle of the boom, in general, carry it to windward in light air, centered in moderate air, and leeward in heavy air. If it attaches to the end of the boom, where it is more sensitive and allows a smaller maximum angle between boom and centerline, carry it centered in light air and ease it to leeward in heavier air as needed to reduce power and feather the mainsail.
Twist occurs when the wind speed is higher at the top of the mast than at the bottom, where the
water surface and waves slow the wind. When close-hauled, twist puts the top of the sail in a puff, which shifts the apparent wind aft at the top and makes the top over-trimmed, relative to the bottom. Twist is always bad while beating. Using the traveler to pull the boom up to windward counteracts the twist and also, by bringing the boom closer to the centerline, allows the boat to point higher. Falling off to a reach by simply easing the main sheet allows the boom to lift. Easing the traveler to leeward, instead of easing the main, prevents this. Only when the traveler car has reached the end of its track is it necessary to ease the mainsheet, and after that the boom vang prevents the main from rising. Many experienced sailors use the traveler when adjusting to changes in wind speed, reserving the sheet for changes in wind direction. Raising or lowering the traveler to trim or ease the main causes little change in sail shape, whereas using the sheet instead would alter the leech tension and thereby change the sail shape. After a tack, when the traveler falls to leeward, hardening it slowly back to windward will keep the sail somewhat full until the boat returns to speed, much like shifting up through the gears in an automobile.