B in Toyota (I've only ever seen it in Toyotas anyway) is for Engine Braking - which is supposed to reduce/eliminate coasting. I think the theory is to replicate what a downward gear change with little or no braking would achieve on a manual (or clutched auto) car so increased revs would seem "normal"... I guess it's not beyond the realms of possibility that the car actually engages B itself while (apparently) in D if it detects no acceleration on a sharp decline or some such?