You turn the line between your two shoulders back and then forward too much together with the line between your two hips.
Imagine a line between your two shoulders, now imagine a line between you two hips. Picture those two lines as they would be viewed from above and the angle that they would form. That is called "
separation angle" or just "
separation". If those two lines move together then the trunk muscles are not stretched and there is less contribution to the forward rotation speed of your uppermost body. I have seen the separation angle for forehands estimated as about 20-30 degrees.
Both rotating the pelvis and rotating the spine/trunk can add to the rotation speed of the uppermost body.
Djokovic uses more separation than most players and he is very flexible. Probably he is not a good model to copy because of his unusually large range of motion for spine twisting. He uses separation when he is not pressed and wants to hit heavier pace, maybe not during slow warm up or slow practice.
In this video, you can see that Djokovic turns the line between his shoulder back farther than the line between his hips. The hips can also start forward before the uppermost body and that adds stretch to trunk muscles. Slow motion at end.
On Youtube use the "
." and "
," keys to do single frame.
If you watch TV broadcasts, higher camera views let you see separation.
Separation, with the same principles of stretching muscles, applies to backhands also.
The ITF seemed to discover 'separation' around 2000 and discussed it. It is discussed in the books by B. Elliott et al, for example,
Technique Development for Tennis Stroke Production. Search:
separation forehand ITF presentation
Separation has usually been discussed for forehands on this forum. I've posted many times with pictures and references.
Warning: Separation involves twisting the spine. Take care with your back and don't do if you have had back issues.
I just found this publication on separation while searching. I read the introduction and summary and it is on the topic of separation.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3761808/