I know some folks were under the impression that reversing the axle worked to tune YYJs and FHZs, but the reason those were tunable was that the gaps were adjustable, and thus the halves could be lined up differently. The one exception to this was some of the older yoyos with big axle threads prior to the universal use of a bearing post - stuff like the Fluchs, the Bape, etc. Those were tunable with teflon tape around the axle, but even that wasn’t because you were reversing the axle, though sometimes that seemed to be the reason; it was because there was enough play between the axle threads and the threading on the halves that the halves could be reoriented a bit.