Changes any value of a layer based on the position
if (position[1] > 500) { value * 0.5;} else { value;}
Changing the opacity of a layer if it crosses a certaing position.
Making a text layer's opacity blink between 100% and 0% every second.
Ensuring that a layer's scale never goes below 50% or above 150%, no matter what animations or adjustments you apply.