Well, that clears that up! I think we did that about 12 pages ago.
Now go watch the video again. Where is Carl looking? He's looking at the damn saw in the wood, just like he should be. Where is that tach? Is it taped to the saw? How often does he look at the tach? How accurate does the manufacturer claim the tach is, and is that with a varying rpm or a constant one?
The technique I used (and described last July) is not something I invented, rather it is something so routine and common it is boring, used in all manner of science and engineering on a daily basis. It's a tool, one you can chose to use or ignore, I could care less. The advantages are that you can look at the rpm over time, for varying lengths of time and at varying portions of the cut as you chose. Which allows you to keep your eyes on the cut where they're supposed to be.