Board Thread:Questions and Answers/@comment-7064657-20130509032922/@comment-5398287-20130509225551

You're thinking of the other Shikai. The first Shikai command Tosen uses painlessly knocks the opponent out by creating a ringing sound which, upon reaching their ears, causes them to fall unconscious. If you actually read the page, you'll see the similarities between that command and Tosen's Bankai. Benihiko is based on his personality just like his Zanpakuto's other powers: it represents his anger and whatnot.