North Koreans put iron staples through the hearts of prisoners in such a way that they are dependant on it for survival. When they step away, the piece of iron will not step with them, and so they will die.
Finn McCool, a Kobold, traded a more crude variant used by warlords in central Asia to the Beagles. It consisted of a crossbow that is sewed to the victim's back. When they step, the iron pin keeping an arrow from firing disappears, and so it shoots the person. Li-Li was ordered to use one on Joshua Valienté when he traveled to Earth West 1,617,524.