A boom brake is a device designed to control the swing of the boom

Boom (sailing)

In sailing, a boom is a spar , along the foot of a fore and aft rigged sail, that greatly improves control of the angle and shape of the sail. The primary action of the boom is to keep the foot of the sail flatter when the sail angle is away from the centerline of the boat. The boom also serves...

 on a sailboat

Sailboat

A sailboat or sailing boat is a boat propelled partly or entirely by sails. The term covers a variety of boats, larger than small vessels such as sailboards and smaller than sailing ships, but distinctions in the size are not strictly defined and what constitutes a sailing ship, sailboat, or a...

. The boom brake acts as a preventer

Preventer

A preventer, or jibe-guard, is a mechanical device on a sailing vessel which limits the boom's ability to swing unexpectedly across the boat due to an unplanned accidental jibe....

when sailing downwind, and can also be used to jibe(US) or gybe(UK) the mainsail in a slow measured action.

Uncontrolled jibes often damage elements of the rig, and can inflict serious and sometimes fatal injuries to crew in the path of the boom or the mainsheet and associated hardware.

The brake usually rides on a line running perpendicular to the boom; when the boom brake is actuated, it grabs the line and either works as a preventer, or slows the boom’s speed while jibing. The brake is actuated by either tensioning the line upon which it rides or using a second line to tension the brake relative to the main line.