The solution to waaaaaaay too many technical problems in the Star Trek universe.

"Captain, the planet's atmosphere is too highly charged to beam down to the surface!"
Geordi:"Maybe if we reverse the polarity of our transporter beam, we can punch through."

"Worf, the ships' shields are absorbing our photon torpedos!"
"Reverse the polarity on a couple of those puppies, and blow'em out of the sky."

"The Doctor's been trying to prescribe trepanation on his last 5 patients! What'll we do, Seven?"
"I suggest we reverse the polarity on, uhhhh..., oh, you figure it out."