It's a common belief that B is an efficient braking solution.
Many car salesmen say that to the customers during test drives, and it's frequently stated even on car reviews: B engages engine brake (true) and charges the battery more than conventional braking (false).
Let's check it:
Scenario: car running at 50 km/h
Test n. 1: switch to B and let the car slow down only by the effect of the engine brake.
Test n. 2: stick to D and press the brake pedal with enough pressure to mimic the same slow down of the B position.
The outcome is outlined in the chart below:
The first test starts at 17:56:20 and ends at 17:56:35
The second test starts at 17:56:52 and ends at 17:56:06
Current to the battery is the red line.
Negative values represent battery charging, generated by braking.
The red area is proportional to the current generated by the recharging effect.
The second area (the one generated by conventional braking) is greater than the first area (generated by engine brake in B).
The second area is 376 units wide vs 296 of the first, so by braking you gain 27% more energy than using B.
If we add fuel flow to the chart, we can see that using B we burn even some fuel.
By looking at the RPM chart, we notice that the petrol engine spins til 20 km/h