It's done to match up engine and drive train RPMs for down-shifting. You don't do it for shifting to a higher gear, only lower. The part of your foot used is dependent on pedal placement but in the Corvette I use the upper edge of my right foot for the brake application and the lower right edge for the throttle. This technique allows you to do your braking for entry into a turn while downshifting smoothly. It's all about keeping the car balanced.