The double dot syntax will animate the elements from when their top edge is visible at the bottom of the screen, until you scroll away and the element's bottom edge passes the top of the screen. It sounds a bit complicated, but it basically will animate a fixed range while the element is visible.
y 200..0 example will basically reach
100px when the element is exactly at the middle of the screen. The animation speed is basically calculated based on the range you're specifying. You can use the other parallax mode and specify an animation factor if you want to control the speed, i.e.
Hopefully I managed to explain it well. Let me know if there's anything that's unclear.