I would assume it's an optical polarizer. Like on sunglasses or LCD screens.

Out the window you go!

If a big magnet protects spaceship earth from cosmic rays, a smaller magnet should protect spaceship enterprise from death rays.

But seriously, my take was always that there was something like an inner and an outer hull, and by polarizing them with opposing charges you might reinforce them so explosions are less likely to blast the outer hull off. Kind of like a magnetic lock around the entire ship.