I think he is talking about the holes in the manifold. Most tuning recommendation is to add some idle rpm on the front carb so that you are not purely on the back at idle. But it is true that the back is doing the heavy lifting, with a progressive linkage that will not operate the front carb linkage at idle and low throttle application. I imagine that notch is to ensure some balance coming out of the rear carb primary venturi's at idle and low rpm. But no expert on that, just a guess.