Nenshi (念糸, Sense Thread) is a basic element of the Hazama-Ryu Kekkaijutsu, which produces a flexible, thread-shaped Kekkai from the center of a Kekkaishi's palm. The thread is controlled largely by the user's will, and can be used simply to bind a target, or to interrogate them with the threat of squeezing them painfully.
Nenshi are also used to string together beads that form collars, which act as seals (sometimes called fūin) for Ayakashi or people with dangerous powers (such as Chushinmaru Karasumori). These seals actively suppress or limit the amount of power the wearer can use, and usually confine Ayakashi to much smaller bodies as well. The application of these collars is extremely painful for Ayakashi: Hakubi likens the sensation to being split in two.