The biggest mistake is using your arms to pull down instead of using your core to actually crunch down. Your hands hold the cable, but the movement comes entirely from flexing your abs and rounding your spine forward. Kneel facing the machine, hands holding the rope close to your chest. Crunch down by rounding your spine and flexing your core hard. Your hips stay locked in place — only your upper body should move. Most people pull with their arms and barely move their torso. Crunch hard and feel it.