The purpose of toroidal flow, i.e., strike-slip motion and plate spin, in the plate-tectonic style of mantle convection is enigmatic. It is a purely horizontal, dissipative flow field that makes no apparent contribution to the release of heat. However, when plate-like toroidal motion is allowed to arise as a phenomenon of non-Newtonian mantle dynamics, it in fact acts to reduce the net amount of viscous dissipation. We show that for a non-Newtonian flow driven by an existing poloidal field (a source-sink field), the generation of toroidal motion interacts with the nonlinear rheology to cause less viscous dissipation than if there were no toroidal motion. With power-law rheologies, the generation of toroidal motion causes up to a 25% reduction in viscous dissipation. With a self-lubricating rheology, which has been shown to induce the most plate-like behavior, toroidal motion causes as much as an 80% reduction in viscous dissipation. Thus, basic non-Newtonian fluid dynamical theory of the formation of plate tectonics shows that toroidal motion is far from superfluous but in fact facilitates the efficiency of convective flow at the surface. ¿ American Geophysical Union 1995