During Makerfaire I asked about this exact thing. The Purex filter provided was pulling ~117 CFM (it has a display that shows filter status and live CFM gauge) which then cause the Glowforge’s exhaust fan to free spin I’d guess 800~1000 RPM. I was told that was okay and not to worry. Which means they have protection diodes on leads to the fan (Or this protection is built-in the fan - I’d have to see if I can see what model fan they are using) to prevent backflow of current. (Maybe?)