I have a Channing III from Alaska with a power-vent. As I understand it, the Barometric Draft Control is meant to keep some of your preciously generated hot-air from going out the chimney (or power-vent as the case may be).
The power-vent (or the standard draft from the heat-difference in the stove vs. outside) may be sucking more air than you'd need to send out just to get rid of the exhaust of the fire. That other gas it would take with it (if you didn't have a draft control) would be hot air from in the stove. The B.D.C. attempts to let the extra air that the power-vent/draft is sucking out be the (less desired) cooler air instead. The B.D.C. tries to be an automatic mechanism for doing it, instead of a manual damper.
Someone with more experience correct me if I'm wrong.
has a picture and description about 1/2 way down the page.