There was some confusion on the framebuilder’s list on how the virtual pivot point works on my frame fixture (and on the Arctos). I took some more photos to try and explain how it all works.
Here are some photos showing how the parts fit together:
This is the seat tube angle adjustment backplate. The adjustment plate rides in the groove slot on some brass pins. There are some machining errors visible in this view, but they don't affect the precision.
This is the back of the adjustment plate. The groove in it is for the locking handle to pass through.
The adjustment plate is mounted on the backplate.
This video shows it all in action.
Click this photo to see a video that shows it all in action. Watch how the BB position doesn't change.