Zanshin is an art of keeping your Ki or spirit focused until the very end of a fight. At the start of practice you bow to your opponent to signify the start of the fight. At the end of practice you bow again to signify the end of the fight.
During a real fight you need to sense an opponent’s aggressive energy before they attack. You then intercept the attack denying them the opportunity to complete that attack. Thereby you beat them. Even after they are beaten do not allow your Ki to relax, you must stay focused. Just because they are beaten doesn’t mean they won’t try to attack you again. Therefore it’s of the most upmost importance that you remain completely focused until the very end.