What is threshold braking?

• When you press firmly on the brake pedal to the point just before the wheels lock.
  Explanation:

  "Pressing firmly on the brake pedal to the point just before the wheels lock is called threshold braking."
• When you press slowly on the brake pedal to the point just before the wheels puncture.
• When you press slowly on the brake pedal to the point just before the wheels stop.
• None of the above.