The way I think of it comes from the old ME degree I got. You can analyze assemblies by the number of degrees of freedom. Three degrees of freedom, like three linkages pinned at the corners is a triangle and the relative positions of each element are fixed.

Four axis, like a square pinned at the corners if you know the relative position of two elements you know the position of the others. This is the basis for most traditional machines.

Five axis is floppy. And usually avoided traditionally.

The synergy drive is a five axis machine mechanically. But has two electric motor generators. By varying the speed of the motors you can control the gear ratio of the mechanical path. Since the motor speeds are indignantly variable so is the mechanical path. You end up with continuously variable transmission with a mechanical power path and an electric one in parallel. And it doesn't suck like pure mechanical ones.

Totally genius.