Why does the slightest touch start a series of stabbing pain sensations?

The mysterious and very distressing symptom called Allodynia is more typical of
PHN than of shingles. Allodynia is pain triggered by some sensation that is not
painful in and of itself but such things as a light moving tuch across the skin can
trigger a sensation transmitted through nerves that don't normally sense pain, but
somehow this will trigger the nerves that do transmit pain sensations.