Taking a short position in a security even though the trader already owns it. The box comes from the safe deposit box where the security is held. A trader may sell short against the box because he or she doesnÂ’t want to disclose ownership of the shares or because retrieving the actual shares is too cumbersome. This technique was formerly used to defer a long-term gain into another tax year, but this was curtailed by the Taxpayer Relief Act of 1997.