A Wartenberg wheel is a device for pricking used by doctors to test nerve responses. It was originally designed by Dr Robert Wartenberg (an American neurologist native of Germany, 1886 - 1956), from which it took the name.
A Wartenberg wheel consists of a wheel with radiating sharp pins and a handle. It is rolled over the skin, similar to how a dough cutter is rolled over pizza or ravioli dough. The pins prick the skin causing (in healthy people) an intense nerve stimulation. Depending on the pressure and sensitivity, the sensation is somewhere on the spectrum between tickle and pain.
Use in BDSM
Wartenberg wheels are popular toys in BDSM. With a normal Wartenberg wheel, the top can roll "lines" across the bottom's flesh. With care, the toy can be used on almost any part of the body. The top has many possibilities of teasing the bottom creatively by varying the speed, pressure, where and how the wheel is rolled. The bottom is typically naked and restrained in bondage to make him/her helpless.
Wartenberg wheels can be bought in medical supplies shops and in BDSM shops.
There are also variants of the toy which have a wide roll instead of a wheel. These will prick entire areas when rolled over the skin.