If you look at the drawing below, you can see how this design is upside down. Virtually all modern and not so modern fretted instruments that utilize a single rod system, bend their rods the other way. I'm sure you'll agree that by tightening the nut, the effective length of the rod becomes shorter and if it becomes shorter, will want to straighten out. If the rod straightens out, it will bend the neck in the wrong direction.
These rods can be effective for very minor adjustments as they will impart some stiffness as the nut is initially tightened.
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