Shape & trim
Boom vang
A tackle from mast base to boom that controls leech tension when the sheet is eased.
A boom vang pulls the boom downward from near the mast base. Its main job is to control the leech when the mainsheet is eased and can no longer hold the boom down.
Upwind, sheet tension usually controls both boom angle and leech tension. Off the wind, the sheet mostly pulls sideways. Without the vang, the boom lifts, the upper leech opens, and the sail develops too much twist. Pulling the vang on closes the leech and keeps the whole sail working.
There is a limit. Too much vang hooks the leech, stalls the top of the sail and loads the rig. In stronger wind a firm vang gives control; in gusts some sailors ease it to let the top open and spill power. On many dinghies it is one of the busiest controls on the boat.