fuelzilla.com (firstname.lastname@example.org) says...
'B' mode isn't a good choice for braking. When you step on the brake,
the Prius will first use the electric motors as generators to provide
resistance and recharge the HV batteries. The mechanical brakes will
only come on either below 7 mph (when the EMs can no longer provide
useful resistance) or when you need to stop quicker than the EMs will
The purpose of 'B' mode is when you're descending a long hill, because
once the recovered energy has brought the batteries to their upper
charge limit, the car will start using the mechanical brakes to prevent
overcharging. When in 'B' mode, in addition to using the EMs for
resistance, it also keeps the ICE spinning, since even though it's not
providing power, it is acting like a big air pump, helping to dissipate
some of the energy without the mechanical brakes. Since the Prius
mechanical brakes are smaller than most cars that size (due to the fact
that they're not responsible for 100% of the braking work), they are
likely to overheat on a long downhill run. It's not much different from
when truck use a lower gear on a long downgrade.
In theory you're not using any gas in 'B' mode, though my ScangaugeII
says differently. It usually reads in the 150-250 MPG range, though I
don't know if the engine really is consuming a small amount of fuel, or
if the Scangauge is just confused, since the engine is spinning.
Bottom line: Using 'B' mode for normal braking and slowing isn't
allowing the Prius to run at its maximum efficiency. If you want to read
more, there's a good write-up here: