Ahh, yes it would be symetrical around the pivot extremes, not the piston extremes.
![:doh:](https://madmodder.net/Smileys/default/doh-45.gif)
But, Russell, I think it's the other way round. It
opens after TDC, and closes before BDC and the vacuum pulls the motor around while the port is closed.
Check out the location of the holes on the inner stationary valve piece below, and notice that the flash marks (and moving port marks) is located below the stationary part. I noticed this lower valve location also in the video, though not as easy to see.
With clockwise rotation of the crank valve is open after TDC and closed during the return stroke. Yes? So the vacuum does the work, I believe, thus the "atmospheric engine".