[quote=@Daxam]
<Snipped quote by Takashi>

Whoops, I used that in the wrong context ^^;

How could Yutaro do that, though?
[/quote]

By using the standard practices of card game tourney rules to his advantage. It is common practice for opponents to cut each others' decks. It is also common practice for them to list the cards they will be using in their deck/sideboard in advance. During the cut, Yutaro can slip an extra copy of a card that is already registered into his deck. Then whenever it is convenient, he can call for a neutral party to do a card count. Even if Zac sees the card count as a cue and uses his slight of hand to remove a card, he won't know what card it is. Likewise, even if he draws the card, he won't know it since it is a card he would be using. The card count would show that he has the correct number of cards, but he will be over the legal limit for the card Yutaro slipped in. Zac would be disqualified for cheating.