One important thing your footwork can allow you to do is to control your contact height. Sometimes you won’t be able to either move up or move back fast enough to be able to make contact in the same place every single time. In these cases, how much you bend your knees (or don’t bend your knees!) will help you control your contact height.
At 0:30 in the video above, we’ve split-screened high speed video of two different forehands I hit. The video on the right is of a ball that is sitting up more as it comes towards me. On the left is a ball that is very low to the ground. Notice that in both videos, I’m standing in the same place on the court. What you’ll see in this video is that I use different amounts of knee bend to help control my contact height, and that the mechanics of my upper body remain the same with both swings. This is critically important, because you don’t want your upper body mechanics changing between groundstrokes if you can avoid it. Especially important is that you don’t want to be bending over at the waist to reach a lower ball.
You can see that when I hit the low ball, it almost looks like I’m sitting down in a chair, but again the key is that my upper body remains upright. If we freeze both forehands at contact, you can see that my upper body position looks very similar even though my contact height is significantly lower in the video to the left.
Just to reinforce, let’s rewind and watch both forehands play in super slow-motion side by side. You can see that the mechanics of these two swings are identical except for the amount of knee bend I use to control my contact point.