Concerning the manipulations with the Nail Board test Yes, if the operator knows his detector very well, he can "tweak" the settings and achieve the planned result, both for the better and for the worse. But just as many of the experienced users of the detectors know at the expense of which particular manipulations this is achieved, and therefore they know what exactly should be shown during the test. And people in specialized forums will listen to what the experienced users will say and show, and not the "jugglers" conducting tests with manipulations. For example, on a Nail Board test, the coil should be at a height of not less than 10 cm from the targets, the coin should be small and consist of brass (VDI = 45, out of 90), nails should not be rusty, discrimination should be completely disabled, if the detector has a display, then necessarily show it (VDI) etc.