The nullifier core was a component of a Romulan warp engine. A slight misalignment in one of the nullifier cores of a Romulan warbird could cause the ship's artificial quantum singularity power source to show through the cloak as an intermittent polarized magnetic distortion whenever the ship was moving. In 2369, the engineer of the IRW Khazara, a sympathizer with the Romulan underground movement, misaligned a nullifier core at Deanna Troi's behest, to signal the USS Enterprise-D that they were nearby. (TNG: "Face of the Enemy" )