The Kaishakunin was the second person in charge of a seppuku. His duty was to help die respectfully the person commiting suicide.
The Kaishakunin position was reserved to only the finest swordsmen, since it was a very precise a delicate job. The person had to help the seppukunin (person commiting suicide) avoid a long agony and a disturbing scene for those witnessing the seppuku.
The cut performed by the kaishakunin had to leave the heat attached to the body by a small strech of skin in order to not let the head roll on the ground. This move was called daki-kubi. Failure was not allowed, since the kaishakunin was putting his own life at risk.
The kaishakunin was designed by the seppukunin or a higher authority (a Daimyo, for instance) and the person chosen could not refuse. Being a kaishashunin was a honor and performing it was a proof of honor and loyalty to a Lord or a friend.
Kaishakunins participated only in 'honorable' seppuku, which happens when a warlord is defeated in battle, for example.
One of the most recent kaishakunin was Hiroyasu Koga, who participated in Yukio Mishima's seppuku.