[Gibbon1]

I found is also somewhat hard to get good info on these systems. It's colored by knowing they are five/six axis machines[1] where you have one or more motors controlling an axis.

A three axis machine is a triangle. Think three linkages pinned. Not very interesting. But a four axis machine is a simple gear train like you have with most battery powered cars. A five axis machine requires two inputs to define the motion of the remaining free axis. Far as I can tell that's what the original Toyota's used. Engine + Motor1 on one input. M2 on the ring gear. Drive shaft is the remaining.

Lot of different ways to skin that cat and they can look mechanically very different.