Quote from
DfT London Airports Night Restrictions consultation
When severe inbound congestion occurs, or is expected to occur, involving airborne holding delays of 30 minutes or more, with at least 20 minutes delay in the inner stacks, it is NATS' practice to land additional aircraft on the assigned departure runway if it considers the landing rate can be increased (e.g. local weather conditions permitting). The procedure is known as Tactically Enhanced Arrival Measures (TEAM). It involves operating parallel final approach streams, but with the aircraft in a staggered formation (i.e. not directly side by side) giving pilots line of sight with the nearest landing aircraft in the parallel landing stream.