Yep, Yikes, Sawtooth and Gigabit G4s all had interrupt switches, I'd hazard a guess that Digital Audio would too. Not sure about QuickSilver or MDD though. Cory's right, the concave switch is the interrupt switch, the convex one is the reset switch.

As for what it does, under OS 9 it would do the same as any other vintage Mac, bring up the debugger. Under OS X, if I recall it performs the same task as the reset button, resetting the machine.