Editing a post because of a missing gerund was pretty unexpected!

But back to topic - in an emergency (about to smash cutter into clamp, body part in imminent danger...) you want the machine to stop with 100% certainty and as quickly as possible. So you won't trust motion controller firmware, cutting power to driver power supply probably won't stop the machine immediately as you need to wait for output caps to discharge, and removing driver enable might not brake motors. Could cut power to drivers on output side of PSU but need chunky and reliable relay as you are switching high DC currents (possible contact welding?) and still lose braking. Could put something (TTL gates? Another relay?) in line with step signals - four of them, in my case.

This is getting a bit silly! In my case, the Pilz device sends e-stop to CSMIO (manual says this has very fast response) and cuts mains to driver PSU (via secondary relay). These use high-reliability n/o Pilz contacts. I am also using the auxiliary n/c contact (not high-reliability) to switch the enable signal. This is for convenience as the drivers are effectively active-low on the enable input so I need to put +5V on them to disable. So, I have two not-absolutely-reliable stopping mechanisms but which are independent so one should work when needed with cutting driver power as a slower-acting backup, but not clear at the moment if I shall get motor braking. Safety is such a compromise...