quoting : "he TAC or Thrust Asymmetric Compensation system helps the pilot control the yawing effect of the Boeing 777 when it suffers an engine failure. When the system senses the thrust level on one engine differs by 10 % or more from the other engine, it automatically adds rudder pressure to minimize the yaw. After a few seconds, the TAC applies sufficient rudder to make it possible for the pilot or auto pilot to center the control column. The TAC can be manually overridden by making manual rudder input. "
hope that answers ur question