Well, you should incorporate all of those aspects into your swing when hitting a tennis ball. You should have a good knee bend, rotate your upper body, and have a loose arm. If you try to swing really hard you will tighten up and actually hit slower. Just try to be loose and use rotation and knee bend to power the shot.
hmm... so i have to use the legs ><... it's still kind of hard for me to coordinate from transferring the weight of my backfoot to the front foot ><, but I got the rotation of the body right.
There really are no secrets in tennis, it just takes time, I recomend to use the back board to master you strokes, at least warm up on it before you play and you will see a big difference.