Hi
Don't have that pin, but if you look at the bottom board, there may be a switch mounted to it, with a peg extending down from it.
Tapping on the bottom, makes the peg jump up, and open that switch.
If that's your case too, its made to work that way.

very rare indeed! my wish too, is that younger folks get into EM machines, pins, and arcades!
some very cool history there that can't be seen when your whole day is stuck in that damn phone...

Yup, thats it. the bottom was left loose and that allows it to flex enuf when you rap on it, to pop that switch open.
There's nuthin to worry about rappin on the bottom of the pin, its the only thing there.
the bottom mech board is suspended above the bottom.
you cud kick the dog s#!t out of the bottom, and really not hurt the pin.
not that I'm recommending that...