Some machines Sensitivity is actually gain and effects depth like the Garretts for instance. Tesoro it seems to effect only the sensitivity to fringe targets and small targets, my guess by shifting the target range of phase response?
It is handy in crappy ground that has a lot of coke and still be able to pick out more solid targets. I used that on my Tejon in a town burn pit and was able to snag a nice walker half without a lot of ground noise.

Really good questions here something that Carl probably could answer way better than my guessing