The Anti-Gravity Matches

Although you turn the tray of a full box of matches upside down, the matches don’t fall out until you say the MAGIC WORD!  WHY?
Because, although you can open the box and show the matches, the box has been faked by wedging a broken match across the box. Thus, when you take out the tray, upside down, the matches won’t fall out until –
... you squeeze the ends of the tray. Then the matches will fall – and the broken, short match is hidden among them as it falls with them!