you need to wall jump off of spiked walls to pass 500m and 800m (check "parity" and "spike jump" sections here: https://celesteclassic.github.io/glossary)
Thanks! I used this in the first few jumps of the secret area as well.
(dash towards the right wall of the screen on screen one without collecting the orb but still opening the chest)
I thot i was done with this game, didnt know there was a whole ass secret room...
