The Prius uses a planetary gear set to blend power of the engine with that of the two motor-generators.

Both the engine and motors are used at all speeds. Particularly during highway acceleration the entire assembly rotates in the same direction.

This is a great related watch if you have some time to kill: https://youtu.be/KnUFH5GX_fI

	▲	delecti an hour ago \| parent [-]
		That's such a fantastic video. I never totally grasped why hybrids were so much more efficient, because my naive assumptions about how they worked were so simplistic. The real-time graphs he showed were excellent for making his points.