Dynamic Load Factor (DLF) depends on the dynamic loads of different shapes, durations which allow the application of a dynamic force, and the natural period of structure to be subjectoed to the dynamic force.
That is how the energy from impact load is absorbed by the structure.
The references are as follows:
Introduction to Structural Dynamics, John M. Biggs, pp.46-48
Structural Design for Dynamic Loads, Charles Head Norris