B Gear gives you maximum braking, you have high control on brakes it is especially designed for Hilly area & Mountain ranges. Because normal D Gear will have continous shifting according to accelerator. I used B Gear last month when i went to Muree, trust me the control on braking is extra-ordinary.
S Gear will give you super drive, when you need acceleration without interruption of gear shifting. For example you need it while you are overtaking or having race.
Trust me, I really enjoyed its driving in S Gear when i want a super drive with heavy acceleration. Because this is only missing element of this machine.
Everything i mentioned above is according to my humber knowledge.