Yes, it's referring to the GI lights. I have a Bally Champ, and when I power it on from the bottom... the lights don't come on automatically. When I tap the left flipper they come on. I have also seen this on other older EM pins. My guess is it caused some confusion for locations when the lights didn't come right on.
Not sure about the "tapping the bottom of the cabinet" statement. Would like to hear about it from folks who may know.