The sensitivity sets the 'active area' of the triggers, for instance, 100% sensitivity will give you full airbrake with the slightest tap of the trigger whereas at 50%, youll have to pull the trigger down halfway to acheive full airbrake, and say, 20% means youll really have to press that trigger to get full airbrake, but you get the other 80% of the trigger movement to give you varying amounts of airbrake.
hope thats not too confusing, im not particuarly good at explaining things