Dovetail jig – backside overcut

So… I’ve been watching some of the older content, like the wall-hung tool cabinet and others where the use of some dovetail jigs are demonstrated. Specifically, https://woodworkingmasterclasses.com/videos/tool-cabinet-project-info/tool-cabinet-episode-2/.

My question/concern centers around the rampant overcut on the ‘in’ side of the tail board vs. the ‘out’ (or show) side of the tail board. Specifically, time stamp 25:06 (which is what I’d want / expect) vs. 25:21, where there isn’t really a visible base line, but it sure looks like the cuts are uneven in length, and from the appearance, at least a good quarter or third of them would have over run *any* line.

I always kind of wondered “how do you know where you’re cutting on the backside relative to the base line, since you can’t just lean over and peek?”

Has anyone else tried this technique, and what kind of success did you have with it? Did you have to modify it in some way to work around or avoid over-running the base line on the back side of the board?