This technique is Hidan's primary method of killing his opponents in the way of the Jashin religion.
First, Hidan makes every effort to draw blood from his opponent and ingest it. His skin then turns black with white markings which makes him look like a skeleton, a symbol of absolute death. He then draws Jashin's symbol on the ground in blood and stands in the centre. This serves as the link between him and his victim.
Once his sacrificial ritual takes place, the victim is linked with Hidan until he either steps or is forced out of the circle. Any kind of injury that Hidan inflicts upon himself (or the injury they inflicted on him) is reciprocated on his target, be it a mere scratch to a pierce in the heart. If Hidan is forced to step out of the circle, the link is broken and the victim is set free, but all Hidan has to do is to step back into the circle to reactivate the link. Basically, this technique makes Hidan a living voodoo doll.
Hidan's immortality is due to the "successful experiment of the Jashin religion's secret technique", so he can fatally torture his opponents in any way he desires without killing himself, and he clearly enjoys the pain as well. However, he does indeed complain that it hurts like one normally does in pain, after he finished off Asuma.