A good analogy I've hears is, imagine the plates are sand paper, imagine trying to slide the sand paper past one another, the sand paper would catch in places getting stuck. well as the plates slide past each other they catch and get stuck, when this happens the force on each side builds up as the plates are still being pushed by the convection currents, then the pressure builds up until the plates get unstuck, when the do get unstuck they jerk forward to where they should be. it is the release of built up pressure, this jerk that is the earthquake.

I hope that made sense, If not I will try to clarify.