When you “hold up” in bridge, you purposely lose a trick that you could have won. We also refer to this as “ducking” a trick. One reason to duck a trick is to cut the opponent’s communication making it so that one defender has none left in the suit and cannot get back to her partner’s winners in the suit. This happens more often when playing NT. Imagine the opponents have knocked out your stoppers in their long suit. If they have done this while both of them have that suit left, they will be able to set you whenever either one of them gets in. However, if you hold up until only one of them has the suit left, then you can safely lose tricks to the opponent who does not have any more of the danger suit left.