When a belt hook is used to span a crossbow, the power of the bow is limited by the strength of the bowman's leg. The biographer of Pero Niño, the 15th century Castilian knight, says that he "used to bend the strongest crossbows from the girdle".
While leg pressing twice your body weight is considered a good performance for a man in reasonably good condition some gifted athletes can exceed twice that. Dan Kendra holds the leg press record at Florida State University at over five times his body weight.
Being able to belt span crossbows that everyone else needed a windlass or cranequin for would have been a useful and impressive skill to have.