How do placements work?

So chances are my mmr was really low because boy was I surprised by my placements. Last season I finished Silver 2, but managed to get gold in 5s so its fine. But I decided I was going to try this season and went 9-1 in placements, and actually carried the majority of my games (except maybe 2). I win the last one and I find out I deranked 3 divisions to silver 5. Just curious how are rank/placements determined?