Apotropaic effect of nakedness
Many historical references suggest that anasyrma had dramatic or supernatural effect - positive or negative. Pliny the Elder wrote that a menstruating woman who uncovers her body can scare away hailstorms, whirlwinds and lightning. If she strips naked and walks around the field, caterpillars, worms and beetles fall off the ears of corn. Even when not menstruating, she can lull a storm out at sea by stripping.
According to folklore, women lifted their skirts to chase off enemies in Ireland and China. A story from The Irish Times (September 23, 1977) reported a potentially violent incident involving several men, which was averted by a woman exposing her genitals to the attackers. According to Balkan folklore, when it rained too much, women would run into the fields and lift their skirts to scare the gods and end the rain. In Jean de La Fontaine's Nouveaux Contes (1674), a demon is repulsed by the sight of a woman lifting her skirt. Associated carvings, called sheela na gigs, were common on medieval churches in northern Europe and the British Isles.
Source: Wikipedia, the free encyclopedia,