Wesley, you keep saying engine-braking is banned as if we don't all know that, that's twice, go on say it again, and it still won't in any way alter the fact that McLaren call those switches 'engine-braking'.
Which is objectively the correct response to the question posted in this thread, "what does that switch do?"
As to the precise nature of this 'engine-braking' mechanism, that is precisely what people here are trying to guess.
... A task I must say not much helped by your generally abrasive and impolite tone in reply to other posters. How about you try and knock this "lol" crap, and "you are wrong" garbage on the head, especially on specifics where no-one, yourself included, knows anything much for certain.
No-one is saying that "engine-braking" means an old-style ABS/TC active modulation of throttle inputs to prevent locked rear wheels.
The phrase could very well be used to mean controls and systems which prevent the negative consequences of natural "engine-braking" ... in which case you would call such controls, when asked to describe your steering wheel, engine-braking.
It could be selection of off-throttle engine maps, it could be an interesting use of the diff and how it sees the locked state of the rear axles, it could be controls to adjust a slipping clutch mechanism which prevents engine braking.
There is a rotary switch, a toggle switch, and a lever utilised, that gives sufficient scope for speculation.
Either way, the point is, we are now just a bit closer to answering the original query about the switch, "It is to do with engine-braking". If anyone wants to run with that and try to get bonus points for providing entertaining and interesting speculation as to how this operates, they are free to do so, and in a forum like this, that should be encouraged.
"lol", and "you are wrong" do nothing positive to encourage such speculation and discussion. You should be more courteous I think.