I think Jeff is giving the wrong answer "Never" to Michaels' question about "When does this preview show you everything?".
He is right when describing the 'Preview toggle' showing the changes for one panel only, but I think the preview shows the combination for all the panels.
It is easy to test this. In the basic panel move Saturation to -100. (the image turns b&w). Then select Tone Curve -panel. (the image is still b&w).
ps. or maybe I didn't understand the question?
pps. or maybe this was the case when the video was shooted. my version is now 18.104.22.168.