Tectonic drift has to be powered with convection currents. The currents do not work against each other due to slip and no-slip conditions. When a tectonic plate has started drifting into a certain directions, those convection rolls rolling with it also connect with it firmly, but those rolling the opposite way loose the grip. This is due to the fact that the mantle is very close to melting point, so extra friction causes melting, but less friction causes solidation.
The large scale and small scale convection rolls then work together according to this picture:
The small convection rolls found below the tectonic rolls therefore play a big role in inducing the tectonic drift.
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