The NSS is a simple switch that is actuated by a plunger that extends into the transmission. When the transmission is in Park or Neutral, the plunger is extended and the switch is closed. This allows the engine to start. When the transmission is shifted into any other gear, the plunger is retracted and the switch is opened. This prevents the engine from starting and also activates the brake lights.
The NSS is a relatively inexpensive and easy to replace part. If you are having trouble starting your vehicle or if the brake lights are not working properly, you may want to check the NSS.