There are three phases to emergency braking: (1.) Realization, (2.) Reaction time, (3.) Braking. Your vehicle does not begin to slow down until phase 3.
Emergency braking works in three phases: Realization, Reaction time and Braking.
At first, you find that there is going to be collision. You realize that you need to brake to stop your vehicle.
After realization to make brake, you need to take your foot from accelerator to brake pedal. This is reaction time.
Then, you need to apply the brake by putting your foot on brake pedal or kicking it.