As I understand it, the brakes in modern gasoline powered cars often need vacuum and when the throttle is wide open (high acceleration) the engine will generate very little if any vacuum, so the assist which normally provides high braking power is not present and this can drastically reduce the braking ability of the car.
Some cars have a sensor to detect if you are overlapping the throttle and brakes and will cut the throttle if the brake pedal is detected as being pressed. This can frustrate people who attempt to match revs on downshifts in manual transmission cars.
