Sherman thought he was sending an SOS signal to the world by activating an "epirb" -- short for emergency position indicating radiobeacon -- which is a satellite-connecting beacon that resembles a floatable walkie-talkie.
Prof Edgerton speculated that "we are activating the spinal cord to a critical level" close to the level which would trigger movement, and a small signal from "the brain pushes it over" leading to movement.