I have a rule about the beginning of the backswing: “If the torso is moving, the arms should be moving”. In other words the arms shouldn’t move by themselves. The shoulders and arms are connected in the backswing.
If your Single Plane address position is incorrect and your arms are not in the correct place at address, they can disconnect from the very start of the backswing. In this picture below, you can see how being too crouched over at address can disconnect the arms.
When the arms move without the shoulders, they will lift above the plane causing causing the club got come over the top / from the outside. Slices, pulls and top shots can be the result.
Many times “posture” of the back is the problem. Also, standing too far from the ball can also cause the shoulders to reach too far – causing the arms to disconnect from the torso rotation. Improperly fit clubs, mainly clubs that are too short can be a big problem as well.
The ideal correction is to feel “taller” and get the spine lifted which pulls the shoulders back and the arms closer to the body.
In the picture below, you will notice when the back is straighter the shoulders are more pulled back toward the torso. The arms are closer to the body even though the lead arm is still above the trial arm. By keeping the arms closer, the lead arm connects to the body and will rotate when the torso rotates.
One way to feel the proper arm position is to Feel as though the trail arm bicep is close to the chest. It’s not completely against the chest but it is as thought you have a towel under the trail arm-pit.
The feeling of the upper part of the trail arm close to the body allows it to fold as the torso rotates into the backswing. This is the connection feeling you will have at the second part of the backswing movement.
As a word of advice. Always check your address position at least once per week to make sure your starting position is sound. I check my address on every practice session. This ensures that you fundamentally correct before you start practicing your swing.