Sensors at the midway point of the table detect the angle, speed, and trajectoryof the not-exactly-oncoming puck, and instructs the mechanism how and where to fire the disc -- you slap it back, and the same sequence happens on your compadre's end.
In an extraordinary scene, Mr Boyce demonstrated that the angleof the round's trajectory meant the weapon must have been pointed at an awkward angle for someone intending to shoot himself.