Getting a higher division in preseason?

Hey. I was just wondering, if i was Platinum 5 in season 6, and for an example hit Diamond 5 in preseason, will my higher mmr affect my ranking placements - such as if i win 8-10 games? Would i be able to hit Platinum 1, or at least Platinum in general? I've heard that you absolutely have to be placed lower than your last division, but i've seen people not even playing in the preseason and getting a HIGHER division than their last one. Can somebody explain ? :C