You can use a custom expression on the rule in this situation. For example:
not(//my:TriggerValue[. != -1])
Will be true when all TriggerValue fields are equal to -1, and
boolean(//my:TriggerValue[. != -1])
Will be true when one or more TriggerValue fields are not equal to -1.
So you could combine the second one with
Dropdown = Not Applicable
To create another rule to set the dropdown back to the Not Started value when the hidden section is unhidden.