The key has to touch the door physically while you're not holding it.
Viewing post in Meyaku Meka (Pastel Jam 2018) jam comments
Hmmm... I'm still not getting it.
When I'm holding the key I can't drop it if it's intersecting the door, and I can't push it if I'm not holding it.
I managed to do it one time - by jumping in the air, releasing the key, moving to the side, and jumping again to hit the key in mid air and knock it into the door - but is that really how you're supposed to use keys?! And then I fell off the edge of the platform and the level restarted and I never managed to repeat my jumping trick. This game may be too hard, maybe? Maybe not, heaven knows I'm no hardcore gamer, but it's a shame I haven't managed to get any further because the mechanics seemed super interesting, more than many of these games (mine included).