The command sequencer is a device used on starships to control warp drive.
In 2373, [[Quark's cousin finally delivered on a promise to give Quark a ship if his cousin ever became a success. Quark had Rom inspect Quark's Treasure prior to engaging it, believing that knowing his cousin the ship was defective merchandise. Although Rom found nothing wrong, he later discovered that the command sequencer had failed. He could not figure out why this was until he realized it had been designed to fail.
Without the command sequencer, a starship traveling at warp speed is unable to stop safely. (DS9: "Little Green Men")