A fusible link sprinkler has two levers held in place by a two-piece fusible link — solder that melts at a specified temperature. When the levers are in place, they hold a cap on the sprinkler orifice, preventing water flow. In case of a fire, when the air temperature gets hot enough to melt the solder, the link separates, allowing the levers to fly apart.
That lets the water pressure in the system push off the cap. Water flows out of the orifice and hits the deflector. The deflector creates a spray that covers the specified area.