The energy of an impact has to be absorbed by a force
developed within the part multiplied by the distance the
part can deform. Designing flexibility into the part to
lengthen the distance over which the energy is absorbed
greatly reduces the internal force required to resist the
impact. For example, a rigid base made from spring
steel would not have as high a capacity for absorbing
energy as a coil spring made from the same material.
The same factors that affect metals also affect plastics.
As more and more flexibility is designed into a part
subject to impact load, the better the part will perform.