Magnetic torpedoes were intended to pass beneath a ship and trigger near the strongest part of its magnetic field. The British system discussed here was designed around that under-hull geometry. The detonation was meant to occur below the hull instead of against the ship's side. The intended geometry is clear, although the reliability of particular wartime pistols is left open.
Notable Point · 2:31:12-2:31:30
Why magnetic torpedoes aimed beneath the hull
Magnetic torpedo systems were intended to sense the ship's strongest magnetic point and explode underneath rather than striking the side directly.