Urtication (urtica: nettle) refers to those who use stinging nettles to stimulate the skin for sex games, such as spanking and BDSM practices. The active ingredients in the stinging nettle do not spread to other areas but are restricted to the site at which the plant comes into contact. The skin becomes sensitized without the injury that certain types of flagellation can produce.
Skin exposed to nettles will redden and in a short time produce small bumps. The person will feel a sharp hot sting that fades to a warm tingling glow which may last several hours.
Nettles may be applied in various ways. Some lie the stems down and press the hairs into the skin, others hold them in a cluster and tap it against the chosen area, or put them into a bottom's underwear. Men who wear condoms have found that briefly applying nettles to the penis before putting on the condom can compensate for the sensation lost by the latex barrier.
Tracy Mehlin at the Center for Urban Horticulture at the University of Washington was kind enough not to slam the phone down when I read her your question, perhaps because she once knew a guy on a farm who occasionally lashed himself with stinging nettles. "I didn't ask him to go into the details about why he did it," said Tracy, "but there are some people who enjoy the effect."Dan also found out that there's a long history behind the practice:
As it turns out, you're not the first person to expose his "delicate areas" to stinging nettles. The Romans thrashed men "below the navel," according to Rodale's Encyclopedia of Herbs, to improve virility.
Among the Druids it is/was a sacred herb, while Roman soldiers were noted to carry bits of the plant with them into battle to give them courage. The nettle was used by Native Americans in the northwest to keep awake on long canoe voyages, by English herbwives to "encourage" prize bulls during the mating season, and by English mistresses for much the same purpose. And stinging nettles were as common in Victorian era erotica as figging, birches, and caning.And here's what they do:
Nettles cause intense burning sensations in the skin where applied, very similar to the sharp pain of a whip. In fact the family classification Urtica comes from the Latin uro -- meaning 'to burn.' But while a whipstroke comparatively fades rapidly (within an hour at least, with aching for a few more) the hot, intense burn of a nettle can last for up to 24 hours! The sensation takes a long time to cool and you're aware of it the entire time due to the almost electric tingle through the area effected. This is wonderful for longterm D/s scenes or verbal and light play in public.
In addition to this intense longterm burning, urtication can also turning even the most jaded iron bottoms into mewling kittens. Sensitivity to any sensations increases markedly, and in the words of Master Conrad Hodson "a feather can feel like a brush, a light spanking feels like a strap, a light strap or flogger blows their minds." One bottom, inquiring if a nettling would effect a spanking and in answer receiving a few swats with just the fingertips, exclaimed "a four-year-old could have me in agony!"
Here's a panel from an Italian fumetti comic of two pretty girls getting a vicious whipping with stinging nettles:
Here's a nettles spanking gallery.