A properly designed zoned HVAC system includes a bypass damper to allow excess duct pressure somewhere to bleed off. Without this relief, damage may be inflicted on the system.
A barometric bypass only opens when the pressure in the duct is sufficient to push open the damper. By venting this off (often back to the ducts leading back into the system), damage is averted. If your system is designed in such a manner that there is a possibility of all dampers being closed, you need to install a bypass.
These reliable bypass dampers from RCS are full dampers, with adjustable pressure settings, and are available in 8", 10", 12", and 14" sizes.