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.
Bending a crossbow from the belt uses only one leg, so about half the weight of a leg press. On the other hand, a leg press is horizontal, and spanning a crossbow is pushing downward, somewhat aided by gravity.
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.