Originally Posted by E90SLAM
It was technically legal because FIA allows the driver to operate DRS, thus allow additional airflow for Mercedes F-Duct enhancing the effect.
So in essence, FIA cannot ban Mercedes to use DRS because they made it work better than others.
Also take a look at SKY Sports F1 Show, the one previews the 2012 season. Ted Kravis had an analysis of McLaren's Exhaust. Even it looked the exhaust pipe pointing towards outwards of the chassis. But the shape of the sidepod creates an inward flow of air mass and pushes the exhaust gas back to the diffusor/rear wing region. Thus salvage 20-30% (not accurate figure) of the downforce created by the exhaust.
This might help explain it.
That is the principal they are trying to exploit at least, I believe.