This is an old trick actually and didn't originate with Tesoro's. I recall doing it with my White's Classic models, too. In a detector that is audio modulated, like the Tesoro's, it's possible that weaker signals, like those from deep targets, cannot overcome the audio signalling limit of the circuit.
(This is technically called the 'audio signal threshold', but we'll not use that term in order to avoid confusion.)
Now, where was I? Oh yes, when this happens to weak signals, they aren't given an audio response that you can hear. They exist, but you wont hear them, or they may be at the very edge of the limit.
Supertuning, or hyper tuning as some call, it is a way to overcome this "lower audio signal limit."
In essence, what you do is increase the threshold to well beyond it's normal 'low hum'. This has the effect of adding a boost to the audio response signal (NOT the transmitted signal, as some have said), thus giving some of these weaker or edge signals a chance of "punching through."
I like to use it in a mild form for nearly all hunting, as it gives a sharper response to nearly all targets. I find it especially helpful in ID'ing foil. In scenarios where really deep targets exist without too much trash, you go all the way with it and max out the threshold control. Together with increased SENS levels, it could spell the difference in such places as plowed fields and dry sand beaches.
For normal hunting, just set it at about 1 o'clock on the knob face. As far as I know, all Tesoros with a threshold control will "hypertune," to some degree.
