Essentially just a bar over the end of a ratchet to reduce the force required. One way to do it with spanners that have an open end and a ring end is to place the ring end over the nut/bolt, and then with another spanner use the ring end and locate it in the open end of the first spanner so it locks in. You've just doubled the length of the spanner meaning you've halved the force required to crack the nut.