How does the barrier taunt work? In chapter 709 of One Piece, Bartolomeo has been seen blocking Elizabello II's king punch with his barrier, while shouting "Barrier". In the translator's comment it stated that this is a common taunt in Japanese school yards. Now how does this taunt work? How would I use it if I were in a Japanese school yard?
According to 日本語俗語辞書 ("Japanese Slang Dictionary"):


  バリアとは、子供が汚い物や人に触れたときに囃して遊ぶときの言葉。
  Barrier is a word that children say jeeringly when they touch an object or person that is unclean.


According to the explanation further down on the page: children say this to someone who has touched something unclean to stop the uncleanness from "spreading". 

As an example, consider a child who has stepped in dog feces. Having done so, the child will try to chase his friends around (and touch them with his hands like in a game of tag, not with his feet to literally get the dog feces on his friends). If, however, all his friends say "barrier" and trace out some sort of pattern with their hands, the child who stepped in the dog feces will have no choice but to abandon the chase (and remain "it", which I suppose is humiliating to any child).

The dictionary seems to think that this was based on people imitating Ultraman. Seems plausible, I guess.

For an example of someone barrier-ing, the interested reader might like to have a look at episode 5 of Oregairu, around the 6:15 mark.



Note: Since I have never been on a Japanese schoolyard, this answer relies entirely on secondhand sources. An answer from someone who has been a kid in Japan would probably be superior.