When you add this validator to a transition and trigger the transition, the add-on checks if the user has changed the value of the selected field on the transition screen. You can identify the field that needs to be modified with a red asterisk suffixed to the title of the field. If no change is found, then an error message is displayed. When the validation is conditional (below), the asterisk is not shown.
To execute this validator based on the result of a groovy expression see Conditional execution using Groovy expression