There are some misconceptions about the Mark 6 exploder. It was not possible to 'switch' between magnetic and impact settings. The exploder was designed to detonate by magnetic influence or impact. It worked very poorly. Some captains, in violation of orders, had their torpedomen take apart and disable the magnetic feature of the Mk 6 before the start of their patrols. This was not reversable.

Later, after tests at Pearl Harbor revealed that there were impact detonation failures, light alloy components were used to make an improved model. I don't know the details of how well these worked or if they retained the magnetic influence feature of the original Mk 6. Finally, a completely redesigned exploder was produced. It was an electrical design and eliminated the impact failure problem. It had no magnetic influence feature. I believe it entered service about 18 months after the outbreak of war.

Even after the Mk 6 exploder problems were dealt with, problems cropped up. The electric torpedos seemed to have many problems, as they were new, and were manufactured by contract, by companies that had no previous experience with torpedos.

This, of course, is a over-simplification. :)