I did not notice a big difference in the slide adjustment. Not by eye anyways. It is was just slight adjustments in the mid for me. It is more of a fine tunning than anything.
It is measuring vacuum.
For example: After doing it by eye, the bike ran fine. I got the tool and checked it like they do on youtube. I could hold the gas at 1/4 throttle, according to the dial on the tester, I was pulling a 5 on one side, and a 8 on the other. I have no idea what the numbers mean, or what was a good number. So I made both sides match at 7..... LOL... no joke. I fig if I was off I would notice. It did not come with instructions or anything. Anyhow, I did the idle the same way. I was not off by much, but I did noticed a difference throughout the low and mid. Not huge, just seemed to run cleaner.
I can see how it may mask a problem. But I think if you have a air leak or something, you will see other signs rather than being out of sync. If one slide is way up, and the other way down, and the meter says you are in-sync, I would trouble shoot something else.