What is ATR Automatic Thrust Restoration in MD-83?
The MD-80 series has two systems that can increase thrust in case an engine fails during takeoff:


Automatic Reserve Thrust (ART)

This system is independent of the throttle position and can increase the thrust of the remaining engine to the maximum rated thrust in case of an engine failure. It can be turned off with a switch on the glareshield, because it must not be used when using a FLEX takeoff thrust.
Automatic Thrust Restoration (ATR)

This system will use the autothrottle servos to increase thrust up to the go-around thrust setting after an engine failure.



  Thrust Recoveries for Engine Failure During Takeoff
  
  In takeoff mode, two separate systems attempt to provide maximum available
  thrust when the performance of one engine significantly differs from the
  other. These systems are automatic reserve thrust (ART) and automatic
  thrust restoration (ATR).
  
  Automatic Reserve Thrust (ART) System
  
  The ART system combines features of the digital flight guidance computer
  (DFGC) and the JT8D-200 fuel control to provide maximum rated thrust in
  the event of an engine failure during a normal thrust takeoff. Upon actuation
  of the ART system, thrust is increased without throttle movement by the
  opening of a solenoid-operated fuel valve in the engine fuel controls of both
  engines.
  
  The ART system is READY when the airplane is on the ground, the ART
  switch is in AUTO, either slat is extended, both engines are operating at or
  near idle, and the ART system self-test is complete. The ART system is
  subsequently armed when the N1 on both engines reaches 64% RPM.
  
  The ART system is actuated when the DFGC detects any one of the
  following: 30.2% differential in N1 RPM, invalid N1, DFGC failure,
  electrical power loss, or manual DFGC switching. Upon actuation, the ART
  system increases the EPR of the operating engine(s) from normal takeoff
  EPR to maximum takeoff EPR (an increase of approximately .05 EPR) by
  opening the solenoid-operated fuel valve in the engine fuel control. Once
  ART is actuated, the maximum takeoff EPR limit is displayed on the thrust
  rating indicator, and the EPR gauge. Once actuated, the ART system is
  latched (ART fuel valve remains open) until the ART switch is moved to OFF.
  
  The ART has an automatic self-test feature. If the ART system fails the selftest, the ART INOP annunciation illuminates. Dispatch is allowed with a failed
  self-test; however, the ART switch must be moved to OFF to disable the ART
  system.
  
  The ART switch, with AUTO and OFF positions, is on the upper instrument
  panel. Two annunciator lights are on the center instrument panel. The
  READY light indicates ART has successfully passed the self-test. The ART light
  indicates the system has successfully activated.
  
  Automatic Thrust Restoration (ATR) System
  
  The Digital Flight Guidance Computer (DFGC) automatic thrust restoration
  (ATR) is a feature, separate from the ART system, that increases thrust under
  certain conditions in the event of an engine failure during takeoff. Once
  activated, the ATR will unclamp the throttles (if the autothrottle system is
  engaged) and move both of them equally until one of the engines reaches the
  Go-Around EPR Limit.
  
  The ATR is armed if:
  
  
  The flight director pitch axis is in takeoff mode,
  The airplane is above 350 feet radio altitude, and
  Both engine EPRs are below the Go-Around EPR limit.
  
  
  After arming, the system will activate if the differences between the engines
  are greater than or equal to 0.25 EPR and 7% N1 (in the same direction), or
  (for DFGC models -930) the airplane’s vertical speed decreases to less than
  zero for 5 seconds. In these cases, the throttles will unclamp (if the
  autothrottle system is engaged) and move to the Go-Around EPR limit.
  
  If ART is armed and the ATR is activated, the EPR limit will be the
  maximum inflight takeoff rating reduced by the same amount that the ART is
  designed to provide. This correction prevents overboosting the engine if the
  ART system subsequently actuates.


(MD-80 FCOM Sec. 18 - Power Plant)