From the QRH
Engine Overboost
A significant thrust overboost capability exists which could be useful in emergency situations. This overboost capability should only be considered when ground contact is imminent. In an emergency situation ‘firewalling the thrust levers' should be considered. Overboosting the engines when the situation is not sufficiently serious unnecessarily shortens engine life and increases the potential for engine failure. This condition could result in an EGT or N1 exceedance. LAND AT NEAREST SUITABLE AIRPORT.