I used to really struggle with this stroke, so I'm not bragging. But it really can be easy. Have a partner hit you a medium high ball and let it bounce. Now, keep it in front of you, ("overhead" is just a figure of speech), you might even want to take it only slightly above your forehead, and snap down on it using your wrist. Later try hitting some from a higher point, and then taking some before the bounce. Take some from behind the baseline, but don't hit down on those, hit with plenty of net clearance. Think of the stroke as being in your hands, and you'll be effective even when not in the ideal position. Forget all that stuff about, "the power comes from your core", "the knee bend gives the speed". How can that be? Even if it does, it doesn't need to be so complicated. With my little wrist snap, I can bounce a short ball over the fence, and you don't need more than that.