What is "B"

The effect of B is similar to downshifting with a manual transmission. It adds engine drag to regenerative braking. I use it on big hills, mostly. It's especially handy when you have more hill left and your battery is nearly full

 

To clarify, the engine braking is there whether you use the brakes or not in B mode

 

Well covered in the manual.
Its for engine braking on steep hills, but also reduces your regenerative braking.
As a general rule, if light to moderate pressure on the brake pedal doesn't control your speed on a steep decline, "B" is an option. Cruise Control will also use it in certain circumstances.

 

It's used to slower the car down by using the ICE without giving gas and electric. Just use the air inside the pistons chamber to counter act to the drive shaft for the speed reduction. It has a braking effect and thus called 'B'. However when driving at this gear, mpg will badly affected. So don't use it unless you need to slower the car without using the brake. Actually you still need to use brake to stop the car at last for a full stop.

 

One more thought on B.
I've had my '06 Prius for over 4 years and never used B.
Of course we don't have any mountains in Kentucky.

 

During heavy bumper to bumper traffic jam, you can try. Almost no need to use the brake to slower down the car. Try next time. It's fun. You will feel relax and stress go away. Of course you must still need to pay attention.