A windsurfing boom is an attachment to the mast that helps pivot the mast around and also helps trim the sail. They normally are in the form of a wishbone boom, which is shaped like its namesake. The boom attaches to the mast between waist and shoulder height, and curves out to either side of the sail. The boom meets again at the tips, usually at the peak angle of a triangular boom.
The mast to which a windsurfing boom is attached freely rotates thanks to a universal joint attachment to the board. The boom itself is responsible for controlling the rotation of the mast. The wishbone shape allows a rider to position the boom to either side of their body, giving them a greater range of travel over a single armed boom.
There was a time when windsurfing booms were tied onto the mast. This was during the formative years of the sport, when it was still adapting surfing equipment for windsurfing use. Today, the boom clamps onto the mast, providing for a much firmer attachment. The rigidity given by a clamp attachment is necessary for many of the tricks performed by current windsurfers. For instance you wouldn’t try to turn your entire rig upside down mid-air if you thought that instead of moving the rig, a good hard tug would simply cause your boom to pull free of the mast.