Phases in the Life of a Brake Pad Wear Sensor:
Phase I - Lots of pad between sensor and rotor.
Phase II - Sensor barely makes contact with rotor under firm braking. Brake light does not flicker when running along, but will come on under firm braking.
Phase III - Enough pad material has worn now that part of the sensor is wearing away too. Brake light will flicker running along.
Phase IV - Sensor is worn completely away. Brake light does not come on at any time.
Beware: Do not mistake Phase IV for your brakes having had a miracle recovery!