With the torque converter split, you can see what does the work. Although this looks very complex, it is quite simple. On the left is the impeller/forward shell, which is tied to the crankshaft. The impeller puts fluid in motion and feeds it to the stator (on right, central hub), which accelerators fluid flow to the turbine/rear shell on the right. The stator does not move, but instead vectors fluid flow to the turbine. Fluid slippage is why we can idle at a traffic light in gear and not stall the engine.