The kinetic energy is stored along the entire spring. From the bend, to the stirrup hook, they are tapered, so that the energy is progressive. (High Performance springs in competition cars are called Progressive Springs) Any minute defect (As John stated, called a stress riser) along the surface can cause a failure when the spring is compressed.
It is blasphemy to say this, but visually compare the lock springs of our beloved Parkers to the spring in a "Best Grade" English gun, and it will be immediately evident the attention paid (or not) to these little bits. 'Theirs' look they're polished to a fairthewell compared to 'ours'.
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