Shield oscillator

Shield oscillators were mechanisms that protected space ships from magno-grabs. The Doctor's TARDIS had one, and the Eleventh Doctor once disabled it in order to allow Clara Oswald to fly the TARDIS. Coincidentally, at that moment, the ship was attacked by a magnograb, and the TARDIS was consequentally destroyed. These events were later negated by the Doctor. (TV: Journey to the Centre of the TARDIS)