[Jeff_H]

Use of the traveller is about twist in the sail. If you sight up a sail underway you will notice that a imaginary line drawn from the luff of the sail to the leech will vary in angle as you move up the sail. This variation in the angle of attack of the sail is called twist. Upwind, the mainsheet, traveller and backstay adjuster (on a fractional rigged boat) controls this twist. The more vertical the pull of the mainsheet and the tighter the sheet, the less twist that you have in the mainsail.

In light air, you want more twist. The telltales on the leech of the main should all be flying and to do so requires quite a bit of twist. Moving the traveller to windward changes the angle of the mainsheet, making it more horizontal, and allows more twist.

In heavier wind you wnat to blade out the sail(flatten the sail, reduce the angle of attack and eliminate twist). Moving the traveller car to the centerline,or even below the center, and tightening in the outhaul, halyard, backstay adjuster and mainsheet really flattens the sail and gives the boat a lot less helm and a lot more control. When reaching the boom vang controls twist in the same manner as the traveler.

Jeff H