OK I found this really interesting
https://the-race.com/formula-1/gary-anderson-explains-mercedes-suspension-trick/
If true then its the suspension lowering that is effectively stalling the drag making the car quicker on the straight whilst allowing a generally higher downforce for the rest of the circuit
Provided this is mechanically achieved which I'm guessing the fia must have checked it seems perfectly legal
Why are red bull so pissed and why don't they copy it... well apparently they simply can't get the same result due to the high rear rake on the car and that is not really possible to change that in a short time as its one of the first and most fundamental design decisions in the car.
If that is what's causing the differential then looking at the circuits coming up its going to be tough for red bull
A new engine with improved output on top of this and Merc / Hamilton will be in with a real shout despite the remaining GPS initially looking to favour red bull
Red bull seem to be making a lot of fuss about a flexible wing but surely the fia have scrutinised that now so if it is the suspension making such a difference it really does seem a very clever innovation