It's genius in it simplicity. The car doesn't actually have a transmission and uses planetary gear sets and two electric motors to fake one.

The gasoline engine is used as the "baseline" power, and the motors then either take power away from it to charge the batteries, or complement it to help in acceleration, or just bypass it completely.

When the car is running in pure electric mode, the gas engine is stopped, and the motors work together to spin the output shaft at the desired speed.

When the car is using mixed power, the gasoline engine is working at a desired power level, one of the motor/generators is adding additional torque, and the second motor is used to make up the rotational difference between the sun and planet gears. This blending of gasoline and electric allows the wheel output speed to be independent of the gasoline engine speed.

When the car is charging the battery, the gasoline engine is run harder than it needs to be to move the car, and one of the motor/generator units is fighting against it, using the excess torque to generate power.

So, there is no transmission, no "shifting", the engine and mgu's are permanently connected to gears and simply spin them at the right speeds for the desired power and speed output.

This is standard since decades in non-electric train engines. https://en.wikipedia.org/wiki/Diesel–electric_transmission

That is incorrect. Toyota's Hybrid Synergy Drive is able to power the wheels with either 100% electric, or 100% gas, or a combination of the two using planetary gears. This allows the use of a more efficient engine running the Atkinson cycle, with the electric motors filling in for the reduced low-end torque.

At highway speeds the engine is driving the wheels directly, which is more efficient than converting torque to electric power to drive an electric motor. Honda's two-motor system runs much like the transmission you linked, but again drives the wheels directly at highway speeds.

I must have misunderstood the GP then, sorry :(

The Toyota system seems fairly different than a diesel-electric train system to me. As I understand it in a diesel electric system the three components (diesel engine, generator and electric motor) all do one thing the whole time and there's no batteries. In the Toyota system however the electric motors can flexibly act as either motors or generators. The gas engine in Toyota also doesn't need to always be on to provide power.

Toyota's hybrid system differs from trains in that there's a direct mechanical connection from the engine to the wheels, boosting efficiency by not requiring all of the power to go through the motors.

It's really devilishly clever. You have to see it to appreciate it.

This site has a nice overview: http://prius.ecrostech.com/original/PriusFrames.htm

This kind of mechanical device where there are more than two shafts are always cool. Classic video about differentials from 1937 (!): https://www.youtube.com/watch?v=yYAw79386WI

Traditional automatic transmissions use a set of planetary gears. The idea is that you can spin the ring gear and planet gears and different rates, which will alter the speed of the output shaft.

Traditional automatics use a two input shafts capable of spinning these pairs at different speeds. A series of clutches select different pairs of ratios for each "gear". A four speed auto might have two ratios for the ring gear and two more for the planet gears.

The Prius transmission takes this idea, and uses electric motors to vary the speed of these gears to create a continuous change in output speed. It's brilliant because it's so obvious when you know the trick. The end result is something that seems really complex, but is arguably even more simple than a manual transmission.

It also eliminates most of the components of an automatic that are prone to wear and breaking, which is a huge boon for reliability.