Can someone tell me exactly how the Ranked Flex Queue matchmaking system works?

Because at the moment, I don't quite understand it. I either get a bunch of silvers/low golds on both sides and roflstomp (I was Gold 1 in S6), or I get a bunch of high golds/plats on both sides. Now, I understand that there is a separate Flex Queue ranking, and I do understand that it tries to match up players in a certain range without deviating too much. However, if that is the case, then does it take your past rating in Season 6 into account? If it does, then how is it possible to climb? I'm currently in my Silver 3 promos, but I don't know for sure if I'm gonna be playing vs actual Silvers or high Golds/Plats from S6. This means that I could stay stuck in Silver, even though my "mmr" has me play against high Golds/Plats because they are within my "mmr" range. ...Wait, is there a Flex Queue MMR? I'm so confused...