This thread had a lot of discussion and several reference links on your questions.
"Hitting-shoulder transversal extension/flexion in forehand", OP, albesca
http://tt.tennis-warehouse.com/showthread.php?t=423053&highlight=Chas+Tennis+forehand+elliott&page=2
Elliott explains that in the take back the shoulders turn back
farther than the hips. This angular difference between the hips and shoulders adds stretch to some trunk muscles. That stretch is used in the forward rotation of the swing. Several references are in the thread above. This issue along with the
stretch-shortening cycle are discussed in Elliott's book
Technique Development for Tennis Stroke Production (2009), B. Elliott et al, available only from the ITF store I believe.
He also discusses the use of
internal shoulder rotation in the forehand. ISR is an axial rotation of the upper arm at the shoulder joint. ISR rotates the entire arm - around its axis - when the arm is straight. It often, but not always, is used in the WW forehand according to Elliott's discussion.
Search Internet:
A Roadmap to a Hall-of-Fame Forehand - Part 9: An Anatomical Comparison of the Federer and Djokovic Forehands
You need to take high speed video of your forehand. 240 fps is a completely adequate rate and 120 fps might be also for the forehand. 60 fps can be very useful also if your camera has a fast shutter so that the motion blur is small. Unfortunately, when 60 fps is put on Youtube or Vimeo it may be reduced to 30 fps by dropping every other frame. But you can examine all frames yourself at home using Quicktime.
Compare to high speed videos on the internet.
https://vimeo.com/63687035