Hello @anon55398716
“Simple paralax effect” get archived because of fixed position of background image. It means that background image on that area will never move, while all other parts of site are moving (and section with paralax are moving too, actually). It is how you would see the “content” from moving train window.
For archive same effect on the section that you want, you will have to make all elements inside it - position: fixed. In the same time, you have to make sure all other sections will have z-index higher than fixed content.
Hope I was able to explain.
Regards,
Anna