Hmm.. What Scarbs say makes sense, but I still think N12ck is right too.
From pictures of the Rear wing it looks like the mercedes style ducts are opened when DRS is closed. Meaning there is flow continuously from the rear wing to the diffusor via the end plates and not the big visible tube(just like N12ck said). Should mean "moving" the downforced gained from rear wing to diffusor which means slightly less drag with same downforce? While closed the large tubing from rollhoop directs air thru the orange->blue and pink tubes on scarbs sketch.
When DRS opens another duct opens on the rear wing main plane (lower green) and forces air down towards blue/pink meaning more diffusor downforce and less pressure on rear wing?