If you put it to "B", it's equivalent to putting a conventional transmission into a Lower gear (1st or 2nd gear depending on speed) to create engine resistance or "engine braking"; by keeping the engine's revs up, it uses the engine's compression to create resistence to help slow the car down without using the brakes (all this is done with your foot off the gas pedal BTW).
the downside is? engines are not normally used to slow the car down, that's the brake's job or in the prius' case, the generator/electric braking and the brake pads themselves, therefore, in the long run this can lower your compression ratio thus reducing your engine efficiency and power output.

In other words, brakes are for stopping/slowing, engine is for moving

Just keep it in "D".

hope this makes sense,
if not , please dont hesitate to ask

gj