Not sure what is the technical name for the switch. But it's basically the red one that you hold in with your right thumb than press the lever to start the machine.
Mine popped off while @ a job cleaning 2night, the red button came lose, so did the small spring on it, couldn't figure out how to put it back on. The machine still worked fine, was wondering if i've damaged anything or is it something i should be concerned about.
You should be able to screw it right back in. Make sure you put the spring back in as well. Any difficulties, call the Watsons at Challenger Pad Systems - 877-446-5887. If you can't reach anyonne there (they are sometimes on voice-Mail) try Lee Watson's cell phone - 903-918-5706.