Mixed Mode
Currently, LHR uses one runway for departures and one for landings, so as to spread the noise of approach and departures. The alternations depend on a weekly rota and, of course, the wind direction.
Mixed mode allowes either runway to be used for take offs or departures as required.
UK govt to trial mixed-mode operations at Heathrow
Currently, in the early morning, both runways are used for landings simultaneously due to the large number of long haul arrivals. Not to mention that the airport cannot operate
During the main part of the day, if there are ten a/c waiting to depart but only three due to land. The ten have to queue one behind the other and the 'landing' runway goes unused.