Explain "B" features?

I'll give you a 'quick and dirty' answer, do a search for "B-mode" on this site and you'll find this and similar questions asked and answered many times over. Be sure to look at Hobbit's excellent and very detailed explaination of B-mode for all the gory details.

B-mode regenerates at a modest amount, but not nearly as much as is possible with braking manually.

But, in the mountain situation you describe the overall gas mileage won't be affected. Eventually the battery SOC will peak out and you won't be able to regenerate more either way. The advantage of B-mode in that situation is that it will delay the need to use the friction brakes for a while longer and will reduce the amount of wear on the friction brakes as long as you're in B-mode.

Also, B-mode is not 'silent'...many will attest that the RPMs will rev very high (near 5000rpm) once the battery SOC peaks out...you can't miss it!

In all other conditions (rare complicated exceptions you shouldn't worry about) B-mode is the least fuel efficient way to brake and really isn't indicated.

 

Here's Hobbit's Article:
http://techno-fandom.org/~hobbit/cars/b-mode.html