Each block has a Position: Sticky and is inside a absolute parent element which is positioned on top of the background. This is only nessesary if you want your sticky stuff to pass between section.
Once your page is setup relative to the viewport, you can use the IX "While Scrolling into View" interaction to move your objects. Check it out in the top left corner. I think it'll make sense when you see it.
That's It!
Hope this makes sense. If not, please let me know in the comments.