What did the Big Red Button on IBM mainframes do?

According to "IBM System/360 Model 65 / Functional Characteristics" (A22-6884-3):

EMERGENCY PULL Switch

Pulling this switch turns off all power beyond the power-entry terminal on every unit that is part of the system or that can be switched onto the system; therefore, the switch controls the system proper and all off-line and shared control units and 110 devices.

The switch latches in the out position and can be restored to its normal position by maintenance personnel only.

When the EMERGENCY PULL switch is in the out position, the POWER ON pushbutton is ineffective.

-- System Control Panel / Page 13

http://www.bitsavers.org/pdf/ibm/360/funcChar/A22-6884-3_360...

I had a mainframe guy once tell me they had a red button that would call home for tech support. Another big system he worked on would trigger a halon flood. And on another it was just a off switch.

My favorite of the 'do not push' buttons on the front of the mainframe was the IMPL button, stood for 'Initial Micro Program Load' and it forced the loading of the micro-code into the CPU.

Party button. If you pressed it, a disco ball would drop down and the latest disco hits would play.