Originally Posted by talmy

Actually, the previous poster was wrong. "B" stands for engine braking. Normally the brake pedal gives regenerative breaking except when almost at a stop. The problem comes with very long downgrades where the traction battery fully charges and the conventional brakes have to be used. Using B forces the ICE to absorb the energy (usually spinning with fuel off). It's a bit strange because if you step on the gas the engine slows down!

You can switch to B at any time, but shouldn't use it except on very long downgrades as the regenerative braking energy is reusable while the engine braking energy is lost. At least it saves the brake pads.