The change tracking tab is used to determine the conditions that cause automatic recomputation of the expression. These conditions affect how often the computation on the Expressions tab is performed. So when u configured in a change tracking tab as whenever property is missing recomputation will start using back chaining when the target property is missing from clipboard.
Backward chaining allows for a computation to advance even when the value of an input or parameter property is not available. This capability is provided by the Rule-Declare-Expressions rule type and the Property-Seek-Value method working together, and can be seen in standard flow actions including VerifyProperty and VerifyPropertyWithListing.