Flair handles back pressure by limiting the number of Smart Vents that can be closed at one time. Flair will never close more than 1/3 of the total vents in a system.
This has shown to be safe by a Lawrence Berkeley National Labs study. To go further, we used a 100% safety margin over their conclusions.
During Setup, when you add your thermostat to Flair, you'll be asked for the total number of vents in your system. This is the total of all vents, not just Smart Vents.