In my spindle manual it says that the machine in which it is used shall have an enclosure as chips and debris of broken bits can be dangereous when flying around. In the industry, such enclosures have a sensor that is connected to the safety circuit and that realizes when someone opens the door of the enclosure which then signals the CNC controller to pause the program and stop all motors.
– Source: Omron G9SE Safety Relay
Unfortunately, “pause” is no available hardware input on the buildbotics-derived Original Series Onefinity Controller, it has only “estop” as input, which makes you loose your positions and program state 
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Regarding automation of the router, the thread is this one:
The topic title is misleading, it is pin 15 (“tool-enable”) on 25-pin I/O port and the command to stop the milling motor is M5.
In case you retrofit this router relay, you might be interested in using my optimized tool change routine. See also How to install the new ‘tool-change’ routine on your machine.