@Timotheedle Quote from the book he uses: "In order to sustain the F#, the thumb may be brought from behind the neck to press the sixth string. This technique, although unusual for the guitar, is common for the cello." From "The Solo Lute Works of J. S. Bach."
I did it on this old video ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Wf8Rhp2hj4k.html, now easier with 8 strings ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-IcRf5Ux7WSA.html