How does a fusible plug work in a hrt firetube boiler?

On every boiler there are a number of safety devices. The safety relief valve, Low Water Fuel Cutoff. If the safety valve and the LWFC fail to do their job pressure and heat will build up in the drum of the boiler. The fusible plug acts as a last line of defense. The plug has a material that will melt at 400 degrees (usually tin) and when that melts away it creates a way for that built up pressure to escape causing a whistling sound. It's not enough to completely relieve the pressure but simply acts as a final warning for you to evacuate the building and/or area.