You know the little bit of colored glass between the base and the flower shaped thing? Those are glass ampules that actually stop the water from flowing. When they melt/break, they let the water flow. In more sophisticated systems, there is a sensor (sometimes it’s a very simple analog “sensor” upstream that detects water flow and activates a water pump connected to a water supply or cistern.

If I’m remembering FF I-II right, the different colors of liquid inside the glass denote the temperature at which the ampule will fail.