A kicker (or kicking strap/vang) is a sailing control line and tackle system that exerts downward force on the boom to control the shape of the mainsail. It keeps the boom from lifting—especially downwind—and controls sail twist, allowing for adjustment of leech tension and boat power.

Key Aspects of the Kicker:
Purpose: It prevents the boom from rising when sailing off the wind, keeping the sail from twisting at the top.

Function: Tightening it decreases twist and flattens the sail (ideal for strong winds), while easing it allows the boom to rise, adding power (ideal for light winds).

Usage: Essential on dinghies, where it is adjusted constantly, and on larger boats, it is often a rigid, telescopic, or hydraulic strut.

Terminology: Also referred to as a "kicking strap" in the UK or a "boom vang".

Benefits: It helps reduce heeling by reducing the draft and makes reefing easier.