Originally Posted by
Larsenex
With this particular shifter shown above, can I shift gears using my right index finger while 'standing up' and ascending a sizable hill?
Yes, the main advantage to integrating the shifter into the brake lever on road bikes is to allow easy shifting while standing. It's more difficult to simultaneously stand and shift with old-school road shifters on the handlebar ends or especially the bike's downtube. However...
While putting hard stress on it will it still smoothly change gears? My mountain bike is 'fair to poor' as I need to change the derailleur and chain rings, under heavy ascending load it can sometimes slip.
...it's not good practice to force a rough gear change under heavy chain tension. Modern drivetrains tolerate it pretty well, but it's still good to alter your power stroke during the shift to allow it to change gears under lower tension.
But even more importantly, you should try not to get yourself into a situation where you're desperate for the next lower gear. Downshift
before you're mashing slowly at the cranks.