Open. This doesn't force air, it simply opens and closes the top vent.
Is that the way you've used it Troy?
Says in the instructions:
"3. On a lit grill, secure the lid and close all the vents EXCEPT FOR THE TOP
VENT. Remember: Tiptoptemp must be able to control all the airflow."
It would
seem what they're doing is having all the air, intake & output, run thru the top vent......
Not sure said the slightly skeptical part of me......
Reason I say skeptical is I'd hate to have stale smoke stalling inside if the exhaust vent was too pinched.............