> Fourth, inside the top box Big John rigged a float from a toilet cistern. If the box was flooded with water or foam, the float would rise, completing a circuit.

The "accelerometer" was another device:

> Fifth, beside the float was a tilt mechanism built from a length of PVC pipe lined with more aluminum foil; inside hung a metal pendulum held under tension from below with a rubber band. [...] Once this was armed, if the bomb was moved in any way, the end of the pendulum would make contact with the foil, completing a circuit.