I am rank 3 in my bronze league division in Starcraft 2. I just played a 4v4 quick match with 2 gold players and 1 silver player on my team, and 2 golds/2 silvers on the other team. And I wasn't the worst player on the list post-game, either. When will I be moved into Silver league?
Battle.net keeps a hidden rating called "MMR", which is similar to an ELO rating used in chess and other games. When you win a match, your MMR increases. The better your opponent, the more in increases by. When you lose a match, your MMR decreases in a similar fashion.
In order to be promoted,
- Your MMR must elevate so that it's well into the next league
- Your MMR must be "stable" so bnet can have confidence that you will stay in the new league once promoted
And note that even if you meet these criteria, the promotion itself may not happen right away. The internal statistics in battle.net are ever-changing, and as such not all calculations happen in real-time. For example, you may win 50 games in a row against gold leaguers and not be promoted. Wake up the next morning, play one game and be promoted.
The best guide to how this works is on Team Liquid: http://www.teamliquid.net/forum/viewmessage.php?topic_id=195273
You need to get a win loss ratio better than 50%. Your Position in your Division isn´t really a clue. There is a lot of math behind the ladder system including the bonus-pool etc... If you really want to get deeper into this, try this wiki
There you will find a link to this chart in the official battle.net blog. Maybe these two articles will help you more than I can ;-)