Just to be more specific: the Automatic Reserve Thrust (ART), combines features of the digital flight guidance and the JT8D-217 engine fuel control unit (FCU).
When the ART system detects a 30.2% differential in N1 rpm-( this is not the only condition that actuates the system)- thrust is increased, whithout throttle movement, by the opening of a solenoid-operated fuel valve in the FCU.
The system is armed at 64% N1.
The toggle switch has Auto/Inop position and is located on the central instrument panel, under a red metal cover.
BD