In Elliott's book he discusses on the forehand that the shoulders should turn back farther
than the hips so that some trunk muscles get pre-stretched for the swing (search: stretch-shortening cycle). He discusses the desirable angle range for the difference between the shoulders and hips - for an effective twist of the trunk.
Technique Development in Tennis Stroke Development
(2009), B. Elliott, M. Reid & M. Crespo
I believe that this book is only available from the ITF Store.
Watch Djkovic to see a large angle between the hips & shoulders. Also, to see the use and timing of pre-stretch in the swing.
The term 'unit turn'
for this overall motion may mislead some players into turning their shoulders and hips more the same amount, reduce stretch and lose pace.