They require much more active computer control to work though so they aren't logically simpler in my opinion, but mechanical simplicity is what makes the biggest difference.
There are certainly better more approachable sources though so I leave this as an option as the info is there, but can take a moment to understand.
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.
Also, YouTube helps. Search for “Power Split Device” or “e-CVT”.
Regarding other hybrid tech, it’s a bit of a crapshoot. I basically use ChatGPT with search enabled to have sources, and then ask how the system works. It’s never in the marketing materials, I guess most don’t care.
My takeaway was the same as some of the other posters here, the system is an engineering tour de force. Simpler than the gas version, proven reliability, and efficiency not too far off the theoretical limit of gas-burning drive trains.
Seeing how simple the whole mechanism is, I totally get why the empirical reliability is so good.
Not to mention that the lecturer shows and explains things very clearly.
Highly recommended!
https://www.youtube.com/playlist?list=PLeFzfl0Q8rQUOHwFNOMTu...
He goes into the practicalities of owning/maintaining/repairing as well as the engineering.