braking energy conversion?

Usually in a hybrid, there is an electric motor supplying power to the wheels. The engine may not necessarily drive the wheels directly. It can be operated at its most efficient RPM to drive a generator, and the generator used to drive the electric motor(s).

Certain types of motors can also make very good generators. By using the electric power generated by the motor to store a battery, this puts drag on the motor which effectively slows the car down.

This sort of "regenerative braking" is only effective at relatively high speeds. At city driving speeds, and for safety reasons, traditional friction brakes are needed to bring the car to a complete stop.

The responses so far seem to be only partially correct. I don't personally know all of the details of a car such as the Prius but the link below gives some additional information.

It's an electric engine working backwards basically.