It detects electricity pretty much. I’ve set one off twice. First time i tried to cut formica laminate with aluminum in it, the metal creates an electric charge so it kicked the stop.
Second time I was cutting a flat pvc sheet and the static electricity from the plastic kicked it.
I know a guy who knicked his finger with one, it just barely broke his skin.
You can turn the stop off with a key if you want to cut off your finger.
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