Quite the love fest between Curious and Mad dog. I think if you really know the throwing motion from the serve, its not only arm but whole body, legs and shoulder rotation to essentially throw your arm. If you have an understanding of the throwing motion from your shoulders then you can make optimal use of the unit turn, its basically same 'throwing' motion and you cannot throw your upper body/shoulder rotation without turning which utilizes your core and don't think you need any more core strength than what you have now cause its just basically a turn and your core needs to be flexible. Next you need to make use of your largest muscles, your legs to assist the core for rotation.
So the athletic stance in tennis can be found in other sports but what makes it different is our movement is more side to side instead of forward backward. Its awkward plus having to coordinate everything else with the racquet, so which is why rec players take a long time to learn and feel its tiring but its really not, they are just not used to it. Yeah you need strong tennis legs but that comes with playing tennis. I think the best way to describe footwork in tennis is - you hit with your legs.
I think Denis Shapovalov is a good player to watch since all his strokes are like throwing motions. Another big thing to help playing tennis with the entire body, believe it or not is not to think about the racquet, just think brush over net to the other side.
Bending knees slightly is fine for rally but why more knee bend is important is because it allows you to move faster because you are in a more loaded position to spring up. Also another reason is to get to low balls.
Strokes start @ 3:00