In light to moderate air, most boats sail upwind best with the traveler car above the centerline. This allows them to position the boom on the centerline without exerting too much mainsheet tension which would close the leech.
With a conventional traveler car, you need to release the leeward control line to bring the car above the centerline. Before tacking, the control line must be recleated and, during or after the tack, the new leeward control has to be released.
A Harken windward sheeting traveler car solves this problem. Windward sheeting cars have the control cleats built into a special mechanism that opens and closes the leeward cleat automatically during a tack.
As you're going upwind, the leeward cleat is open so you can draw the car above the centerline. When you tack, ignore the car — the old leeward cleat closes, the car stays where you left it, and the new leeward cleat opens so you can draw the car above the centerline on the new tack.
In heavy air, when you want the car below the centerline, the windward sheeting car works like any other—just ease the control lines to position the car, and it will move to a reciprocal position on the other tack.
There's a windward sheeting traveler for every boat from dinghies to 13.7 m (45 ft). If you already have a Harken traveler, you'll find a retrofit kit available for most.