James, good to see you around. I hope your tennis is going well. We should hit sometime soon. Now that you mention it, I really wanna see if you bend your knees when you hit your backhand these days. hehe.
Anyways, back to the topic, yes, it is essential to bend your knees, and never at the waist. If you look at Federer or Gasquet, when they get low balls, their knees will almost be touching the ground.