The start of the backswing, called the takeaway, is critical to positioning the club properly on plane, and to enabling a full turn of the shoulders and hips. A number of things can go wrong in this small space.
You can break or hinge the wrists too early, causing the clubface to open and ensuring a slice. You can move only the arms while keeping the shoulders locked in place, resulting in a weak turn and a loss of power. On and on it goes.
If you can get the first foot of the swing correct, it’s much easier to reach the top of the backswing in good shape. Any time you’re struggling with your swing, the takeaway is a key fundamental to check.