It's the functional equivalent of the 'L' gear in most other automatic transmission cars. It uses the gas engine to slow the car down rather than having you hit the brakes. It also isn't as powerful as engine braking in a conventional car, though, since when you 'coast' in the Prius, it uses the electric motor 'in reverse' to recharge the battery. The 'B' mode is really useful only in cases where you are going to be going down a slight, but long, incline, and you know that the battery would be charged to full by the bottom. 'B' stresses the engine system slightly less by using engine braking early, rather than waiting until the battery is full, then using HEAVY engine braking (which isn't so good for the engine,) or resorting to using the physical brakes (which it isn't good to 'ride' in any car.)