You can first try to understand what the phrase means to begin with. Short selling against the box is like an act of short selling all the securities which you already own but it does not close out the existing long positions. It results in a neutral position where the profits and losses are equal and net to zero. I can put it in an example for you. Supposedly you own 50 shares of a company and it can be any company and you request your broker to sell short 50 shares of that particular company. This way you will conduct a short sale against the box and you will have a long position in one account and the short position in another account. This is pretty much a restricted practice and most people don't like to indulge in it often