The act of putting a strap on on your back and proceeding to fuck two people, one in front and one behind. Humping between the two swinging like a clocks pendulum.
Large (pendulous) breasts swinging from side to side (like a pendulum) under a loose fitting shirt giving the appearance of two kids fighting under a blanket.
An action not unlike a Newton's cradle where balls of a similar size crash into each other and bounce back.