It's actually unclear what technique he's talking about. I suspect "scale" has a particular meaning in the context of weight lifting, which might make sense to his viewers but which does not mean anything to the rest of us.
If I were to guess, I would say it means to "adjust" the weight that you are lifting, so that you lift either progressively heavier or progressively lighter weights in a set. See "An Idiot's Guide to Progressive Strength Workouts", if you're not familiar with the concept.
Either that, or to adjust the weight for the number of repetitions you are doing. Again, it's not clear which he means, but it might make more sense if I watched the entire video.