I bought one for my wife in 2011, it's been a great car for her, except for the STIGMA, so I have wanted to put that very bumper sticker on it ever since we bought it. "NOT A LIBERAL"
As far as how it works, the gas engine comes on or shuts off whenever it wants to. It depends on the state of charge in the battery and the demand being placed on it, like hard acceleration, gas engine comes on, coasting down a hill at 70 mph, or sitting at a stop light, the gas engine might shut off. , that's how it saves fuel, by shutting off when not needed and running on electricity. The electricity generated from the braking is minimal, but at least it saves on brake pad replacement.
Her last car was a GMC denial, it got 13 mpg, cost $56k and fell apart at 100k. now when she drives across town to play bridge, at 50 mpg, it's no big deal.