Did you try reading the manual?
B(rake) is not related to winter at all. It's for driving downhill to recharge battery more efficiently. You can find "Winter driving tips" in page 412 of the English manual 
I live in mountainous northern Spain and my experience is that the B position is for short downhill road and B position also charges the batteries (rapidly) and when batteries are full the engine disconnects from both the charger and the drivetrain, resulting in a free fall of the car and it is necessary to use the brakes excessively and they start to smell (burn). All in all a dangerous system for mountainous driving.B-mode is only for engine breaking when driving on a long and steep downhill road. It can be engaged or disengaged at any given moment, even when you're driving. Toyota hybrids are clever enough to automatically enable engine breaking (B-mode) if the battery is fully charged and you almost never need it.
The downside of using B-mode is that is revs the engine and reduces the regenerative breaking, increasing the fuel consumption. In winter conditions there is no difference in the car behaviour.
That is not how B mode is designed to work - take the vehicle to a Toyota main dealer as if its behaving like that then something is wrong.I live in mountainous northern Spain and my experience is that the B position is for short downhill road and B position also charges the batteries (rapidly) and when batteries are full the engine disconnects from both the charger and the drivetrain, resulting in a free fall of the car and it is necessary to use the brakes excessively and they start to smell (burn). All in all a dangerous system for mountainous driving.
Regards,
Rene de Wit