Hey Andy, hey Trevor, hey BJ, hey Derek, hey all,
the correct pin to hook up a relay that switches the milling motor on and off is not L1 (pin 2) or L2 (pin 1) but Tool-enable (pin 15). ‘Tool-enable’ is controlled by the commands M3 (milling motor on) in conjunction with Sx command (where x>0) and M5 (milling motor off), which are commands that are already there in every g-code program.
If you used the ‘L1’ (workpiece flood coolant on/off) or ‘L2’ (workpiece mist coolant on/off) outputs for this instead, then you would always have to make sure the M7, M8, and M9 commands are inserted into your program, whereas M3/M5 (motor start/stop) commands are already included 
.
I think what you remembered Andy was the other day when Branden @WildMtnCraftworks discovered that ‘tool-enable’ ceases to work when he sets ‘tool-type’ to VFD, and he was looking for alternatives for switching coolant and dust collection on (he had problems using the DRV/UPF outputs of his VFD for driving Arduino inputs). But since Trevor does not use a spindle/VFD, he has no reason to set ‘tool-type’ to a VFD, so there would be no reason why not to use the ‘Tool-enable’ (pin 15) functionality of automatic switching on and off for the router.
Using ‘Tool-enable’ for switching router on and off automatically by the g-code program is also what @satoer does on its Remote Control Panel (newest version see satoer.com).