A foldback quasar engine was an alien power source of unknown origin. One was found by the Eleventh Doctor when the TARDIS brought him to the Royal Albert Hall in 2010 during the second Proms in his honour. He promptly stopped it from exploding from an overload. They could be disabled either by sonic screwdriver or by improvising a bio-damping feedback transducer. The latter dissipated the force of impending explosion through the Wi-Fi at the engine's location. (Doctor Who at the Proms (2010))