Enter your backside stance, squat down, wind up and push up hard while spotting the target. Keep your arms open until your chest faces it. Wrap them in and spin around once, while still rising. Spot your target again and then quickly turn around for a hook kick. It should feel like entering a stepover hook or Cheat 720 from midair. That is when the Backside 900 and come in handy. It's basically a Backside 900 that is canceled after the first spin into a stepover hook kick.