While responding to an emergency, a car is tailgating the ambulance.

Comprehensive and Detailed Explanation From Exact Extract:

In situations where a vehicle is tailgating an emergency unit, tapping the brakes briefly is a standard defensive driving maneuver to alert the driver behind to increase distance. It is less risky than slowing dramatically or pulling over, which could endanger crew or delay response.

Speeding up could reduce your reaction window or increase accident risk. EMS operators must follow safe driving practices per NFPA 1002 and DOT Emergency Vehicle Operation Guidelines.