I thought that it (slit) work like this:
The aim is to reduce pressure difference between three area so teams bleed higher pressure air above rear wing into outside of RW endplate to accelerate air of this area to reduce ambient pressure (outside of RW EP) just like multi element wings (slot gap wings) that bleed in high pressure air above wing to underside of wing to increases the speed of the flow under the wing to reduce flow separation.
Please correct me if i am wrong.
