It can be accompished with how the brakes "bite" on initial application and how the lever "feels". Okay, you can apply enough force on the stock system to reach maximum tire traction. It takes a certain amount of pressure on the grip to do that, and it takes a certain amount of time to develop that pressure and a certain amount of control to keep it right at that point of maximum traction. Until they get clamped down not that much slowing takes place. If the pads create greater friction when they are not completely clamped, when they first contact the rotor, there is greater deceleration when it has greatest effect on stopping distance - when you are going the fastest. At 60 mph you are going 88 feet per second, so cutting even an eighth of a second off the time means the brakes are doing their thing about ten feet sooner. That helps regardless of whether you have ABS or not. If it is non-ABS, being able to maintain the brakes right at the point where lock-up is imminent means you get maximum deceleration. If you have to let off and re-apply to keep them under control some of that time is spent at less than maximum. Add to that the effects of multiple applications - where stock brakes often fall short - and new rotors and pads can decrease stopping distance.