RedBull's Mysterious Ride Height
Essentially it's the pull-rod rear suspension and the way it works with the current diffusers. Obviously diffusers are highly sensitive to ride height because their effectiveness is largely dictated by the air pressure under the car.
Put simply, the cars must run more ground clearance than they want in qualifying (so when race fuel is added ride heights are still manageable) which ruins the performance of the diffuser as the gap between track surface and car floor becomes too large to create the ideal pressures. All the top teams are supposed to have broadly
similar designs with the exception of Red Bull, whose diffuser is so compromised by their unique rear suspension layout that it is significantly less effective.
Components of the pull-rod suspension occupy the same space through which diffusers like to be fed air. That's the disadvantage of the RBR suspension. The advantage (when properly exploited, as it is here) is that the rear bodywork behind the sidepods is noticeably slimmer and less fussy, supplying the rear wing with cleaner air and allowing it to work much better than would otherswise be the case.
It's a standard balancing act. Do RBR gain
more downforce from the optimised rear wing than they lose
from the compromised diffuser? The cars ultimate pace suggests they're not losing out, but more importantly this system seems to give them a huge advantage on low ('light') fuel.
Because they rely proportionally less on their diffuser to create downforce, and more on their rear wing, they lose less overall downforce when running big ride heights (downforce provided by the wing isn't affected by ride height, downforce provided by the diffuser can be crippled by it). And the effect is compounded.
With their super-effective rear wing doing more to push the car low to the ground Red Bull are losing less of their diffusers functionality than teams with less wing. This might explain why McLaren (allegedly suffering specifically with a lack of rear downforce) seem to be particularly hobbled in quali compared to races.