Some call it "ratchet" pinpointing .
Back in the day, to narrow the audio width of target response for defining a more accurate location of center, the technique called detuning was used. Detuning forced the electromagnetic field to become less sensitive to the target, thus reduced signal response width.
Many of today's metal detectors incorporate VCO (voltage controlled oscillator) into the all-metal pinpoint mode. VCO is driven by target voltage to produce varying audio pitch responses for target pinpointing and identification. Although VCO may be difficult and unpleasant to use on large shallow targets, it can be more of an advantage when multiple targets occupy the same pinpointing zone compared to monotone audio. With VCO, by narrowing the target area significantly, it is possible to hear the pitch peaks of two targets in close proximity more clearly.
If your metal detector has a continuous auto-retuning feature incorporated into the all-metal mode, you must be careful when hovering the search coil over a target. If you initiate Pinpoint while over the center of the target, the whole target may be masked as its response will be tuned out. This is why many detectorists experience "vanishing signals" in the all metal mode when attempting to pinpoint a target too quickly.
The above from detecting world.
