Originally Posted by Aptos Driver

I.E., it's analogous to downshifting on a grade, which I used to do all the time with a stick-shift car (up and down). I still drop my V6 RAV4 from "D" to "4" when I'm descending a long grade.

Originally Posted by Ron AKA

Yes, that is the exact intended purpose. On a long downhill you are going to fill the battery one way or the other and waste energy after that, so it doesn't matter much when you put it in B mode.

I think the mistake is using it in other situations of shorter duration where the battery is not full. Yes, it helps to charge the battery when you put it in B mode, but at the same time it forces the engine to waste energy. It is kind of like throwing the baby out with the bathwater.

Originally Posted by Ron AKA

This is not a well understood function. It is selected with the gear shift. One salesperson told me it was B for Battery. I believe it is really B for Brake, and is intended to be used on long steep downhills where you may shift down in a standard transmission or down in an automatic.

Since the TCH functions like a CVT and does not have discrete gears to select, what it does is change the timing of the engine to make it work more effectively as an engine brake. The engine intake manifold vacuum goes up and the engine pumps air back and forth. This generates heat in the engine and kills energy. It is taken away by your engine cooling system. The advantage of using it, is that your engine does the braking instead of the disk brakes. Disk brakes over a long steep hill can heat up and fade, as well as damage the brakes.

The disadvantage of using it is that it permanently wastes energy (engine heat). It is more fuel efficient to just use regenerative braking. You can do that by using the brakes just enough to regenerate to the battery but not engage the disk brakes. However once the battery is charged, that is no longer an option. Then it is better to switch to B mode to save your brakes.

Here is a link which goes into it in more detail:

Synergy Hybrid B Mode Explained

The simple way to think of it is that it turns the engine into a brake, and forces the engine rpm up to pump air. If you need that braking and have no place to save the energy (battery full), then use it and waste the energy. If you have battery room then it is better to just brake and charge your battery while saving the energy.