Speed & Smarts: Take Control of Your Mainsail – Part 2 – Traveler
The traveler controls the angle of attack of the mainsail in much the same way as hydraulic cylinders control the flaps on an airplane wing. When the plane is going slowly and needs a lot of lift (e.g. to take off or land), the flaps go down to make the wing more curved and more angled to the oncoming wind.