Pulley joint
A joint that links two bodies over imaginary pulleys so that when one descends, the other rises.
The pulley joint holds the sum of two rope lengths constant: length1 + ratio × length2 = constant. Two bodies hang from separate ropes over two fixed points, as if the ropes ran over blocks in the ceiling. Pull one down and the other goes up. The ratio (the mechanical advantage) lets one side move faster than the other, exactly as a real block and tackle trades force for distance.
It ships in Box2D and drives counterweighted elevators, trapdoors, moving platforms and classic physics puzzles. It is finicky: if a rope goes fully slack the constraint misbehaves, so engines often clamp the lengths.