This really needs an animation but I don't have time ATM.
Essentially the mechanism is a crank, but instead of the crank-pin being parallel to the main bearing axis it is angled and extended so that at one end it is concentric with the bearing axis and at the other it has maximum offset .
The orbiting mechanism (OM) is connected to the crank by a spherical bearing which slides up and down the crank-pin. usually the whole crank shaft slide up and down as it is adjusted and the OM stays in the same plane, thus as the relative positions of the crank and OM change so does the size of the orbit.
Often two cranks are used which allow phase variation as well as amplitude allowing ellipses at any angle as well as circles.
Hope that makes some sense