"valueAtTime()" grabs the value of a property at a specific time, unlike "value" which grabs the value of a property as it is now.
You specify time as a number inside the parenthesis.
Creating a delay between layers
In this example, there is no need to parent the layer itself to the main one. We're going to use an expression to grab its Position value.
However, instead of using "value", we're gonna use "valueAtTime()", and instead of specifying a time in seconds, we're gonna use the current time and subtract the amount of delay we want (in seconds).
For the "parentPosition" variable value, use the Pickwhip tool to select the parent's Position property, like so:
And we're done! Any change in Position the parent layer will perform, our child layer will perform as well but with a 0.2 seconds delay. How cool is that?