While attempting to uncover the command that Dennis Nedry had used to disable the power grid, John Arnold attempted to search the whole computer system code. As a suggestion to save time, Henry Wu reminded him that by using the keychecks command, he would be able to directly trace all of Nedry’s keystrokes on the system. The Keycheck system had been placed as a security measure so that they could monitor the keystrokes of the operators with access to the system.During the course of his attempt to cover his tracks, Nedry attempted to turn the Keychecks function off. However, he was not aware that it wasn’t possible to turn that particular system except by manually flipping switches on the main board.
Jurassic Park by Michael Crichton page 228-230 (First Edition, Thirty Fourth printing)