Hybrid "B" shift....

Is the "B" (brake) shift option on the Hyrid an electric "downshift" or is it engine compression. On my previous '06 HH shifting to B did minimal slowing on a hill and was very quiet. On the '08 it sounds like there is significant engine compression, and the car slows a lot more... but the mpg guage shows the engine is off. How does B work on the '08's. Is it safe to use at higher speeds, such as going down a mountain pass at freeway speeds?
thanks