You then took that clicker to your local arcade, and clicked one of the corners of the CRT, that would send a shock through the system and add credits to your game. I believe this was because the CRT was grounded on the same ground lines that the mechanism for physically checking a coin had gone through the system.
Suffice to say, they caught onto this over time, and added some form of an alarm into it. But up until then... Those were truly the best times.
To my understanding some of those early arcade games also had jumpers to control some of the behavior. It could be that a tech set the "free credit on reboot" jumper and forgot to reset it when they were done.