Product review (2 stars): Good idea, inadequate design, very poorly manufactured
This is a great idea that I have used before to good effect.

UNFORTUNATELY...

This PARTICULAR device - essentially an air valve - is so poorly manufactured that it required repair BEFORE it could be used.  The plastic shell is a flimsy, two-piece, snap-together affair that didn't snap together well at all, thus leaving the interior valve flap in a warped shell that did not allow the valve flap to seat properly in either position.

I repaired and modified mine so that it now works properly, but I remain annoyed that this product required pre-installation repair straight out of the box it came in.

Clamps provided are actually just plastic wire ties that are inadequate to hold dryer hoses under pressure.  Buy a real set of hose clamps to install this item and/or use duct tape to keep the vent hoses in place.

Screen is small and will rapidly become clogged with lint that gets through your dryer lint screen.  Install this device so that you can easily access its screen for removal and cleaning.

I strongly suggest you purchase a different brand of this device.

NOTE:  USE ONLY ON ELECTRIC DRYERS!