I made earlier versions of this page, referring to the waist cut?. We fell into the old terminology/tactics trap again: because "waist cut" is not a standard translation, the reference was cleared. However, the current page doesn't describe anything useful, just that the keima can be cut, which we all know, not how to cut it.
At the top, we see how Black must cut in this basic situation. The shape develops to a windmill. At the right, Black's shape is worse, because he has a diagonal development into the cross-cut. See also haengma tutorial for beginners. At the bottom, a different starting situation. Again the move leading to the windmill like shape is good, whereas the left side shape, with the zig-zag development of Black, is obviously bad.
These basic considerations lead to the proverb, or so I thought.