Inertial losses occur when an object with a given mass resists changes in its momentum. Or stated another way, when an object with a given mass resists angular acceleration.
Examples where this may occur:
Accelerating components such as: crankshafts, flywheels, torque converters, clutches, pressure plates, harmonic dampeners, wheels, tires, driveshafts, axles, spools, pulleys, connecting rods, etc.....