As with every puzzle in this area, the answer involves dying. Be sure to pay attention to the key's behaviour when you do. Good luck!
This one was the only screen that stumped me until finding this comment. I believe on a psychological level it's an amazing puzzle, as in every platformer (and every game), right after dying is when you take a breath, relax a bit, and stop paying attention, even if it is only for a half second.
Congratis for the originality on this one!