Not patched, I've been asked about it a few times so I'm planning on making a vid when I have time, but in short:

1. Freeze rocket in tunnel
2. Do a normal jump -> clipped into floor
3. If head above ceiling then wrong clip, slide and go to step 1
4. Floor has potholes in, walk into pothole (I always dash to a specific one)
5. Stand still, then tap a directional key (I'm tapping w in my vids)

I imagine for anyone that's struggling it'll be step 5 that's catching them out. I find the overall process somewhere around 75% consistent and would do it in fg runs, but it is a bit of a pain for IL runs to get the clip quickly.