To generate a good hip drive, the leg must function like a spring. Think of the hips like a kid on a trampoline—they have to elevate above the trampoline before coming down to land on it. That same kid, upon landing, would feel how the trampoline provides an opportunity to redirect the force of the landing into a bounce back up. You could also steal your bounce, if you know what I mean. In the same way, an effective hip drive uses the leg as a tool the hips can leverage to launch into the drive.