An empty leg is a one-way repositioning flight with no passengers on board. Every charter can create one. Say an aircraft based in Delhi is booked from Mumbai to Bengaluru. First it must fly Delhi to Mumbai empty. Afterwards it must fly Bengaluru to Delhi empty.
Those two flights are the empty legs. The operator pays for them whether anyone is on board or not.
That is why empty legs can be sold cheaply. The aircraft, the crew and the fuel are already paid for. A passenger on that flight is extra income against a cost the operator has already accepted. It is not a shop-style discount. It is simply how an unavoidable flight is paid for.
The catch is that someone else’s trip sets the empty leg. Its route, its date and often its departure time are fixed by the charter that created it. You fit yourself to an existing flight instead of booking your own. That is the opposite of what charter normally offers.
Empty legs can also change. If the original charter moves, changes route or cancels, the empty leg moves, changes or disappears too. An empty leg is firm only once the operator confirms it. The closer to departure, the firmer it gets. If you need certainty weeks ahead, an empty leg is the wrong choice.
Empty legs suit flexible travellers who have a direction, not a deadline. For example, someone who would like to be in Goa this week, can leave at a day’s notice, and can live with the trip not happening. For a board meeting on Thursday morning, book a regular charter.
An empty leg is not a lower class of service. It is the same aircraft with the same crew that would have flown the paid flight. The only differences are which way it is going and who pays for it.