On the forehand, for example, landing on the outside foot and pushing towards the center helps to power and accelerate the arm.
In doing this the knees and hips are involved, as well as the lower back.
I have a drill that makes players find the most efficient use of those parts.
Because it is repetitive, you don’t need to think about it. Your feel and instinct will adopt that which is more efficient and feels better, plus upon observation gets the ball in the court.
The drill is to put a cone or can of balls in the center of the court, behind the baseline, and just run and turn, both at the cone and during the hit. You can actually pull the racquet towards you rather than following the path of the ball.
Tennis is simple if you keep it simple. In actuality, it is an easy sport to learn.
That has been the direction of my own coaching efforts.
I guide you so you don’t fall for the complications that are so usual in this wonderful sport.