PHEV discussion is on topic here if people weren’t aware.

This YouTube video describes the planetary gear set used in Toyota Prius and similar Hybrids. Different hybrids (like Volvo or Honda) have completely different mechanisms, but I think it’s safe to call the Prius and RAV4 the king of hybrid/PHEVs right now.

The planetary gear act as a differential going backwards. Instead of a powered shaft powering two wheels going at different speeds (around a turn for example), the Hybrid Powersplit Device allows two motors (EV motor + ICE motor) power a single shaft despite the two motors traveling at different speeds.

Toyota would tune these gears so that the EV motor and ICE engine are operating in their ideal ranges. This, leading to the huge efficiency of the Prius.

http://eahart.com/prius/psd/

This website has a decent simulator to play with as well.