It's been a long time since I was in school and this is far from my field, but this doesn't sound right to me. Isn't the main cause convection currents in the earth's mantle? Is cooling lava pushing the plates apart a cause? I thought it was basically an effect — where the plates diverge, molten rock pushes up and fills in. (Or, is this just two ways of looking at the same thing?)

Slab pull is easier to imagine: a subducting plate, sinking into the asthenosphere (uppermost mantle), pulls the younger plate behind it. Not all margins are destructive, however, so slab pull does not operate everywhere. Indeed, the relative important of push and pull is debated.

You will sometimes see figures like this one, strongly implying that convection and plate motion are strongly coupled. The earth is a complex system so the processes are certainly related, but we don't understand the frictional forces (drag) at the lithosphere–asthenosphere boundary well (e.g. read this primer by DiVenere). These images, while prevalent, are an unhelpful over-simplification.

Note: I'm using DiVenere's copyrighted image under 'fair use' terms. Please replace it with an open version if you find one! (But not one with convection cells :)