As I understand it, he threshold setting changes where the internal generator switches from fundamental to harmonic mode or similar. The threshold you are setting will depend upon the capability of your devices's chip to oscillate beyond its specs; some will go above the default 300MHz and some only as high as 280MHz . The idea as I understand it is to reduce the threshold setting until you get a clean sweep on both sides of the threshold point.
73, Don N2VGU.
Donald S Brant Jr