So Rating is different from MMR? Is there any way to look up your MMR or is it completely hidden?
MMR is hidden, even in the WoW system. It's used simply to match opponents and figure out the correct points to award in a win/loss to help your rating reach your MMR in a less volatile way.
Say you have MMR 2300 with a rating of 500, playing MMR 2400 with a rating of 2400.
If the 2300 player wins, he would get a lot of points (500 rating to 2400, huge underdog)
However, the 2400 team would lose points equivalent to an even match, (2400 lost to 2300, he was a slight favorite)
If the 2400 player wins, he get a decent increase, as a 2400 just beat a 2300.
But the 2300 player loses very very little, since he was 500 and lost to a 2400 (hardly an even match).
Basically you award points based on your ACTUAL rating, compared to your opponents MMR.
you then take a hit or boost to your MMR based on your win/loss.
It's just a fairer way to award points and help high MMR player climb the ladder faster, and doesn't punish a 2400 player for losing to a 500 player who is way above 500 in skill... the system treats it like he lost to a 2300 player, which is more fair to him.