Put something small like a ping-pong ball between your wrists, with your arms extended over and behind your head. Hold it there for 10 seconds as you try to "crush" it between your wrist while keeping your arms completely straight. All of the energy for the "crush" should be coming from your shoulder area, and you should be able to feel those muscles working to crush the ball.
When you do a back walk-over you have to use those same muscles to keep your wrists and hands close together as you arrive at the beam. You shouldn't just "think" about the hands being close together, you should reproduce that muscle effort you learned while standing with your arms in position while trying to crush the ping-pong ball. So don't "just do it", you need to "feel it".