In any mechanical process when heavy weights are involved, it is important that the lifting wheel does not spin out of control if something goes wrong.
Leonardo studied several variations on a locking system, which blocks the wheel's rotation in the wrong direction while the weight is being lifted. The catch presses against a cog on the wheel, preventing it from spinning backwards and dropping the weight.
The idea, originally used when loading catapults, forms the basis for ratchet- locking mechanisms in use today.