I've seen Sonny play a number of times.
Playing behind the
slide is a difficult technique, I'd call it 'advanced' because it requires quite a bit of practice, but its not impossible.
IMO, and after observing Sonny- your success at 'Fretting Behind The Slide fretting (FBTS) will vary greatly according to your gear and setup. You will have the best chance of success after you have developed a light touch with the slide, and mastered both left and right hand dampening.
The basic concept is that if your action is high enough, you can press a finger down onto a fret behind your slide, and the string will have enough clearance under the slide to ring through as a fretted note rather than the slide note. The fretted string must have enough clearance under the string when fretted to ring. Sonny often plays in an open tuning. The best example is the ability to turn any straight across slide chord in open E tuning to a minor chord simply by fretting one half step behind your slide on the the middle G string lowering the major third to a minor third. (I hope that's correct)
Sonny uses that technighe quite effectively to devise little riffs that are successions of notes that are assembled using a subtle rocking motion that gently releases the slide note to the fretted note behind the slide, or frets behind so the note sounds under the slide. This is hard to describe in written word and equally difficult to master.
I've just begun to be able to achieve the technigue on electric guitar after plenty of failed attempts. A lot has to do with the setup of the guitar, and it helps in the beginning (and later) to have some overdrive or compressed sustain to assist you in getting all the notes to carry through. Of course effective damping with both hands helps to get a clean sound.
Barry