What does it take for a promotion?

[Eijolend]

I would really like be able to give a detailed guide on how this works, but we just don't know much about the system.

If you take the several forum threads with this thematic into account it seems like you have to regularly beat players in your league and have an acceptable record vs higher league players.
Also please consider, that for searching an opponent the system can only use your record up to that moment. The system can't know that you're about to win several games you have ongoing. With the possibility of up to 20 games at once it shouldn't come as a surprise that the adjustment for better opponent's feels delayed because only your most recently started matches are matched with your current rating.

I think these two issues, that delay everything, are what most people are experiencing that are on a 20-x streak. This also shows it's very challenging to develop a ranking system that can handle multiple games at once. (the well known rating systems like ELO, sc2's MMR or XBoxLive TrueSkill usually aren't confronted with this problem)