The circuit of a typical model rocket controller is really quite simple.
It consists of a battery pack (the power source), and igniter (that has the basic electrical properties of a fuse), 2 switches and a lamp. (The Lamp shows that the circuit is working but adds enough resistance the circuit to keep the igniter/fuse from blowing.)
When switch 1 is open (off) no current flows through the circuit and it is safe to connect the electrical leads to the igniter.
When Switch 1 is closed (on) and switch 2 is open the current flows from the battery through the igniter through Switch 1 then the lamp and back to the battery. Since the lamp has some resistance, only a small amount of current flows through the circuit keeping the igniter from heating up. The lamp lights and lets us know the circuit is properly connected and everything is ready for launch.
Closing Switch 2 with Switch 1 closed, creates a short circuit. Like water, electricity will follow the route of least resistance and since there is virtually no resistance through Switch 2, the current flows through switch 2 instead of the lamp. The lack of resistance in the circuit causes an overload in the circuit, the igniter heats up rapidly just as a fuse would. When it blows the spark launches the rocket.
