Nice effect.
Fixed than relative:
(Change CSS dynamically on Scroll) in this case the section style from position: fixed; to relative in pointX - is only by custom code (See this wishlist idea)
Basic Example of the concept her.
Opacity trick
The Opacity trick - two images one on each other - (How? with position: absolute; you break the normal flow)
When you scroll the opacity change and you get cool effect:
before
after
With this library:
But maybe you could achieve this by webflow interactions (I never try to create this effect on webflow- see this forum Q)
Anyway - this is more freelancer job - no way to solve this by forum answer (To solve this you should look at the code and try to reproduce idea).
I hope you find a solution 