The principle design feature is that two or more wheels, each mounted on a "swing arm" that forms a bell crank, so the upward movement of the wheel is turned into sideways motion of the top of the crank.
The principle design feature is that two or more wheels are suspended from a common point between them. Each wheel is mounted on a "swing arm" that forms a bell crank, so the upward movement of the wheel is turned into sideways motion of the top of the crank.