Product review (5 stars): Pretty good design
The lid on top isn't what opens when the dryer blows air out the vent.

There is a cap inside that moves up and down in a track.  When the dryer is blowing air out the cap lifts up and when it stops blowing out air the cap falls down and seals the pipe by gravity.

The lid on top does open for cleaning.

Pretty good design.  I like it and it keeps cold winter air out better than my old vent.