If we are talking airplanes here, and not fish, it is the charge that fires the fire extingisher bottles both on the engines and the APU. There is one squib to each bottle and they are electricly activated. When you get a fire warning (talking 737 here), you pull up on the red fire handles in the cockpit to arm the squibs and getting them ready to fire. Then by turning the handle to one side you activate the extingishers by firing the squib. There are two fire ext. bottles for the engines on the 737 and if one bottle is not enough to get the fire out you can select the other one to the same engine by rotating the the fire handle the other way around. If the fire has not gone approx 30sec. after the last bottle is fired, then it is time to start saying your prayers and get the airplane on the ground ASAP, airport or not! Also, when having fired both bottles on one engine, the other one is without fire ext. equipment. The APU has one bottle, and it is not possible to fire that one to any other engine and the bottles for the engines can't by fire on the APU.