www.planeur.net vous propose de tres nombreux documents en libre téléchargement : 
Yaw string is a basic "equipement" of our gliders, that often questions the people out of gliding movement. My answer to such inquiry is to say "this is a low cost sideslip sensor" - with a tongue in cheek...
Well yes yawstring relates to sideslip, but at the end do we really know how much the angle we observe every flight relates to sideslip ?
http://sailplane-matscherrer.blogspot.com/2023/01/on-yaw-string-behavior.html