If memory serves, the only difference between magnetic and impact triggers is that you get to turn on or off the magnetic trigger. That is, the torpedo carries both triggers. If the torp is impact, then the magnetic trigger is unarmed, but if the torp is magnetic, then both triggers are armed.
So, a shot that would bounce an impact torp should also bounce a magnetic. However, by shooting under the keel, the impact trigger won't go off, but the magnetic, which is looking for iron deposits lurking just above the torpedo rather than in front of it, will go off. If you botch your shot and the torpedo actually strikes the ship rather than going under it, you have a chance that the impact trigger could still go off, but it's long odds against. At least that's what I recall, I could have the real-life mechanism all wrong.
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