What is mean by back up torque and when it applies?

Answer 1

Torque backup is generally applicable to low speed diesel engines which are primarily used for high torque applications like tractors and off highway vehicles.

Generally torque backup is expressed in percentage

Torque backup = (max Torque - rated speed torque)/rated speed torque in %

This generally means how much the engine can be loaded when its rpm drops to max torque rpm. Usually all OEM try to give maximum backup torque from their engines. But it is limited by volumetric efficiency at lower rpm for NA engines and for TC engines it can be more.