The Kill switch was a genetic device installed at the base of the brain of all Enhanced Face Dancers by the Independent Robot, Erasmus. When cornered, captured, and ready to be "found out" as a duplicate, the kill switch could be activated by that individual Enhanced Face Dancer to produce instant and permanent death. Since Erasmus was the instituter and installer of the kill switch in the Enhanced Dancers, as a back-up should they ever revolt, set it up that he could activate the kill switch instantly throughout the entire cosmos by using the Thinking Machines tachyon net.
Erasmus was forced into the position of doing just that when on the day of Kralizec, the Enhanced Dancer leader, Khrone, announced to all humans and machines assembled on in the Cathedral on Synchrony, that the time of Enhanced Face Dancer ascension had come. Khrone did this because in the previous year, man and machines had decimated each other's forces, leaving a wake of destruction throughout the galaxies. Only the tens of millions of Enhanced Face Dancers spread throughout human-kind's Old Imperium, and the multitude of Dancers on Synchrony; remained as a coherent and well-functioning armed contingent.
To his utter shock and surprise, Erasmus announced to all assembled, including Khrone, that he could kill all Enhanced Face Dancers by activating their Kill Switches. Erasmus activated the tachyon net, and over the course of the next few minutes, activated all Kill Switches throughout the Known Universe. Immediately, tens of millions of Enhanced Face Dancers died throughout known space, as they collapsed instantly.