alt Hacker NewsFun story: I used to own a Prius, and it turns out they expose the speeds and torque values of MG1, MG2, and the engine independently on the OBDII port.
What this means it that you can set up an app like Torque[0] and add widgets that show you how fast each of the motors are spinning, live, and watch what happens when e.g. the engine starts: MG1 and MG2 both torque the engine forward, MG2 just enough to stop the car from attempting to roll backward in response to MG1's torque through the planetary gearset, and then MG1 spins up with the engine and then stops torquing it once the engine reaches idle.
Battery charging while idling is similar: MG1 turns itself into a generator, fighting the engine and generating electricity in the process. The throttle opens considerably, as if you'd pressed the accelerator halfway to the floor, but MG1 and the engine work together to keep the engine's RPM around ~1,200 so you'd never know it - it's as if you're driving up a really steep hill that stops you from accelerating even though you have the gas pressed halfway down. And then MG2 torques backward to stop the car from rolling forward any more than the Prius's normal "simulate a normal gas car's tendency to roll forward when the user lets their foot off the brake" would have it do.
It was fascinating to watch, and I kind of regret not building an app similar to the parent comment's link that showed what my car was doing in real time with the gears drawn out like that.