Focus on keeping your backswing compact – or in other words short – and execute it slowly. This is in contrast to over rotating your body and/or to do so in a rushed fashion.
Keep your left arm stiff during your swing, including at the top of the backswing. This is in contrast to letting your left elbow bend, which some golfers tend to do as they proceed to the later stages of the backswing.
Allow the heel of your left foot to come off from the ground during the backswing. This is in contrast to keeping the whole sole of your foot squarely on the ground throughout your entire swing.
Focus on keeping the sole of your feet on the ground throughout the swing. Specifically it targets the heel of your left foot during the backswing, which may tend to lift from the ground as you coil your upper body back.
Concentrate on moving the left knee towards the ball during the backswing, or in other words move it slightly down and to the right. This assumes the ball is located in the middle of your stance. This is in contrast to seeing the knee cap move aggressively sideways towards your right foot, or straight towards the right knee.
Concentrate on moving the left knee straight forward during the backswing. This is in contrast to seeing the knee cap move sideways and down towards the ball or towards your right foot, or in other cases towards the right knee.