As we have seen, many of the other teams have moved toward a lower and longer line cooling outlet at the base of the car, whilst shrinking the cooling outlet around the exhaust. This creates another area of low pressure above the diffuser (slower moving airflow exiting from the rear of the car).
McLaren's 'wishbone wings' prohibit the team from following a similar path, requiring them to remove the wings and change substantial areas of bodywork. Under the old regulations this task would be large but not insurmountable, however the curtailment of CFD and Wind Tunnel time makes widesweeping changes fairly prohibitive as the teams cannot then concentrate on refinement too.
The new solution from McLaren is used to better focus the airflow in the region and hopefully help the diffuser to operate over a wider spectrum.