The tool that can help you with this is called LogViewer - see the UserGuide for details. Set logging level to "full" and the LogViewer will display all details on how Aware IM executes and checks the rules. You can search for specific events in this tool. This can be extremely helpful.
Rules attached to business objects are executed by the Rule Engine, which is quite sophisticated. There is some detailed description of this in the User Guide - see the Data Processing section.
If you still can't figure out how this particular attribute is changed let us know.