This is an extremely quick-to-tie knot that makes two loops which can be tightened down around someone's two wrists. While it can be convenient for capture, great care must be taken because of the ease with which the loops will continue to tighten, and the tendency for a single loop of rope around the wrists to dig into vulnerable nerves in the wrist groove. Generally, I view this technique as to be avoided.

Wikipedia Reference

The Mexican Handcuff is a slightly more secure (but much slower to tie) version of the same concept.