If I'm understanding what I've been reading about this, and that's not guaranteed, it's the off throttle that allows them to open the wing mid corner.
The Driver gets off the throttle and then the engine disconnects from drive and revs. The gases this produces are blown over the diffuser to increase downforce and that is what allows the driver to open the wing early.
If they need to lift the throttle completely, I can't imagine them having the rear wing open.