Addendum: Revised and reposted in order to add this relevant information found by cavalaxis:
Pozza, M. E., J. L. Stella, et al. (2008). "Pinch-induced behavioral inhibition ('clipnosis') in domestic cats." J Feline Med Surg 10(1): 82-7.
There has been much interest in using mechanical inhibition for gentle restraint of rabbits, rodents, mice and guinea pigs. In these species, immobility is induced using neck clips or inversion. Although it has not received much interest, there have been reports of immobilizing cats by placing clips along the dorsal midline or neck for short procedures such as blood sampling. The authors have coined the term "pinch-induced behavioral inhibition" (PIBI) or "clipnosis" to describe this method of restraint. In this project, the effectiveness of PIBI was evaluated in 13 healthy cats and 18 cats with idiopathic cystitis (IC) using standard two-inch binder clips from a stationery store. In the first month of the study, 92% of the healthy cats and 100% of the cats with IC responded positively to clipping. The physiological response to clipping was similar to that of scruffing a cat - miosis, ventroflexion of the back, curling the tail under the abdomen. While clips can be placed anywhere along the dorsal midline, the authors recommend the dorsum of the neck as the most effective location. No cat exhibited behavior that could be interpreted as a fear or pain response. The researchers concluded that PIBI can be a safe and effective method of gentle restraint for various routine veterinary procedures, such as blood sampling, vaccinations, and nail trimming.