Much as been written about the TENNIS forehand mechanics from a Swing and Kinetic chain perspective.
Generally there has been a lot of emphasis in loading of the outer leg as part of the initial swing process.
From the many videos that we see of PRO players on the circuits , it is quite apparent this is a very important component.
What about AFTER loading ? Where exactly does the counterforce lead to ?
Does it straight away lead into HIP rotation ?
Or is there another step before that ?
IMO, a lot depends on where and how much time we have
IMO we can do either of both
1. We can LOAD on outer leg and immediately rotate the hip OR
2. We can LOAD on outer leg and drive a stepping action with inner leg BEFORE pushing up AND ROTATING the HIP
Step 1 is effectively anchoring only on OUTER leg and ROTATION straight away.
Step 2 is pivoting and stepping and pushing up into the ROTATION.
IMO much of FEDERER forehand motion is based on Step 2.. In the accompanying pictures you will notice he has not rotated FULLY even after he has stepped with his left and notice how straght the front leg is at point of contact (This can only happen as a result of pushing up action). Notice how STRAIGHT the right hand is as well.
You can easily compare the difference both in straightness of LEFT leg and RIGHT hand with Djokovic's (who use STEP 1 more often) . Both pictures are FROZEN at POINT of contact.
Food for thoughts