The related document logic relies on the Rule: Related Item Exists. There is a checkbox on this rule called Use Related Items for Tasks. Whenever something is found, the true branch will execute on the related item instead of the document in workflow. This is where you can use your property expression. As a bonus, if your related item search returns more than one document each related item will execute the true branch sequentially.
One of the common examples we use in Training (get the sum of all checks that match an invoice):
Warning: when using logic like this make sure to CLEAR properties or you risk creating an infinite loop.
