This was a neat hack in many of the early Macs between the 'big switch' ones (like the Mac Plus and SE) and the 'pushbutton' ones (like the Performas and Quadras).

You could even do it with your fingernail; just push in and twist the power button, and it would stay in forever, and the Mac would automatically boot when you plug it in.

My Quadra 700 has this too