What's needed to clip through the upstairs door? Is it a long jump into the door or into the wall near the door? Is it just a normal jump? A double long jump? is the timing of the ground pound very precise? I've been trying to get it for like an hour now and have had no luck. Any tips would be greatly appreciated.

First you need to get the camera behind you so that you are facing into the door/wall corner. Then, run slightly into the room to get the door to start to close and immediately run at it and longjump into the door/wall junction. It helps to hold left into the wall as well as O on the longjump.

Feel free to join the Discord if you have any more questions!

I tried TASing it, turns out it's a 1 frame trick. The interesting thing is that if you do a regular jump into the door and then a ground pound, you get sent flying through the wall like in the second place run, but if you do a long jump then a ground pound you end up next to the wall like in the WR run.