Thic circuit is placed in line with the brake light's power wire. The user cuts the wire, and solders this in place. The third wire is connected to chassis ground.
When the driver presses the brake pedal, the brake light will turn on, turn off, and then turn on and remain on until the brake pedal is let go - all in about 300 milliseconds.
This circuit helps catch the attention of a tailgaiting driver that might not be paying attention.