The follow-up I was responding to was Steve S's follow-up; not yours. As to say 'I Have One Too', I have what Steve has - A flapper that's installed on the OUTSIDE of the house. (NOT what you have/are describing ...)
I know of what you're referring to but never even gave it a thought because, well, my outdoor (flapper) vent (ie., my vinly vent accessory) works just fine in keeping air from rushing back into the dryer. As long as I keep the lint off the flaps, it stays shut. Also, my dryer doesn't leak into the 'house' as long as the door is shut. I guess there isn't enough 'force' coming from the outside to push any air into the vent. This 'action' would just push the flappers shut.
I think Steve S was just saying what he had (ie., a flapper on the outside of the house.) DEFINITELY, neither of us is 'talking/describing' that which you are describing.