Empty legs are the unsold segments on a private jet. For example, a private jet is chartered one-way from Dallas to Miami and then the plane must be repositioned to Chicago for its next chartered job. The segment from Miami to Chicago is known as an "empty leg" as there is no paying customer on the flight. These non-revenue generating flights typically come at a steep discount - up to 50% off the normal jet charter prices. Moreover, the customer has the further benefit and flexibility of selecting a departure date and time, within a designated window, making these "on-demand" versus "scheduled" flights.
Typically, individual jet operators market these empty legs flights independently through their own website or through newsletters to select members. EmptyLegTweet retweets empty leg availability in order to find the best deals on chartering a private jet.