The lever principle
In a spanning unit, the elements form type 2 levers, like in a wheelbarrow. As soon as an element becomes suspended, the lever can be interpreted in both directions, which balances the element. Usually levers are used to gain an advantage, like lifting a weight with less force at the expense of a longer path of movement. In a leverwork, the levers form a static system, passing the loads on from element to element, creating an equilibrium. Often gravity is involved as a force, but not in every form of leverwork.