Answer
To open the Task debugger go to the following URL:

http://server-name-here/aexhd/worker/Default.aspx?cmd=debugTask

Select the task you want to debug from the Task drop-down list. Type in an appropriate incident number to debug with and click the Refresh button.

This will evaluate the task in context of the given incident and will display the results of each criterion and the overall result of all the criteria, which will help determine if and why a task with the given criteria would or would not fire.

This can also be helpful in troubleshooting complex Incident, Routing, or Notify rules by creating a task with the same criteria as the rule in question, then debugging with this tool.