shangnom

You actually can shift a gripshift while braking. If you use your middle finger to brake and your index finger/thumb to shift, you can go up or down a couple gears comfortably while braking. It takes a while to get used to, but it is possible. You could also always use the other brake for that brief shift moment. There are also other advantages (gearbox related) that might possibly negate the need for you to shift while braking. Mind you, I am assuming the pinion shifts like the rohloff. One reason you might downshift while braking is so that you are in the proper gear once you are ready to pedal (Imagine a downhill section that quickly transitions to an uphill). With these transmissions you can concentrate on braking and then instantly downshift several gears (as many as you want) and be ready in the proper gear. You can even shift at a dead stop (very handy when commuting or if you have to stop suddenly on the trail). If you think about it, pedaling (required for derailleur shifting) while braking doesn't really make sense. Also during really gnarly sections you don't want to be pedaling constantly (might hit rocks or put you off balance) so shifting with a derailleur is not an option. With the gearbox system, you can still shift.