[Mr. Hasselhoff] is using a disposable cell phone to trigger his fireworks. He has wired into the speaker leads for the speaker phone. When the phone rings, the current sets off a thyristor allowing for a battery pack to be discharged into a rocket fuse. These fuses heat up and ignite, so you can use them to light fireworks fuses pretty easily. This is pretty simple and cheap, considering the price of the cell phone was only $10. His next idea was to have it recognize dial tones and set individual fuses off, but that would require a microcontroller and a much more complex hack. At that point, you might as well just build a fully fledged wireless fireworks launching system and possibly add rocket launching abilities too.