The brake usually rides on a line running perpendicular to the boom; when the boom brake is actuated, friction on the line and either works as a preventer, or slows the boom's speed while jibing.
The brake usually rides on a line running perpendicular to, and below the boom. When the boom brake is actuated, friction on the line either works as a preventer (stops the boom from moving in the direction that would slacken the main sheet), or slows the boom's speed while jibing.