The dominant forces that drive plate tectonics are those created by subduction.  Slab pull is transmitted throughout a plate system when subducted oceanic lithosphere remains mechanically attached to its parent plate.  However, detached slabs that descend into the mantle, excite viscous flow that might exert traction on the base of the lithosphere, thereby sucking plates along.

This item and others about Tectonics can be read at Earth-logs in the Tectonics archive for 2002