Okay i got to this room too and had no idea what to do. The fact it's a speed puzzle and you're not told this is rather bad design on the dex's part if puzzles need to be solved a certain way it should be obvious to some degree like a speed puzzle having a times or something to show how long till the door closes stuff like that. I'm not trying to be mean just giving my two cents i don't like it when a puzzle is to vauge to solve