Probably a test of "mixed mode", where either runway can be used for take-off or landing (not sure if that actually includes parallel landings or take-offs), rather than the current dedicated take-off and landing runway allocations.
In theory, it can be much more efficient in LHR
's case, given the layout of runways and terminals. It's always a pain to land on 09L/27R when you then need to taxi round and cross the other runway to get to T4
(ditto for departures from T4
that have to get to 27R). It would be much simpler to always let planes using T4
land and take-off on 09R/27L. Since all the other terminals are between the runways, this is less of an issue for them.
Time flies like an arrow. Fruit flies like a banana!