You're Correct!

Across the line in the golf swing means that at the top of your back swing, you have the club in a position where the shaft is pointing to the right of the target – across the target line – rather than it being parallel to the target line.

Being across the line at the top of your back swing makes it very difficult to hit straight shots accurately at the target, as you will either swing the club down so that it approaches the ball too much from the inside of the target line, or you will attempt to correct the position and by doing this you will throw the club head too far on to the outside of the target line. Either way of moving the golf club back down towards the golf ball will make it extremely difficult to swing the club effectively and consistently along the target line, which is a pre-requisite of straight golf shots.

This position usually occurs from an incorrect takeaway movement. Your takeaway is literally the initial movement of the club head away from the golf ball. When you make your takeaway, you need to keep the club head on plane. Your plane is easily seen if you take an alignment pole and push it into the ground at the angle of the shaft of your golf club at address. If you do this and then take a step to the left of the pole, keeping the club head in line with the bottom of the pole where it is pushed into the ground, you now need to make your takeaway so that the club head follows the angle of the alignment pole.

This will see the club become horizontal at hip height but also parallel to the target line. This is the correct position to achieve on your takeaway. If you are across the line at the top of your back swing, you will tend to move the club head under the alignment pole on your takeaway, so that when it is horizontal at hip high, the shaft of the club, from club head to handle, is pointing to the right of the target. From here, you will end up across the line at the top of your back swing as you complete your swing by turning your shoulders.

A great drill to work on to correct this takeaway position, is to stand in front of a wall and take up your address position so that your backside is touching the wall. Make your takeaway without letting the club head hit the wall, and learn to move the club so the shaft becomes parallel to the wall.

If the shaft of the golf club is aiming left of target at the top of your back swing, this position is actually called laid off. It causes similar issues with achieving directional control and accurate golf shots. The correct position is to be parallel with the shaft at the top of your golf swing.

If the shaft of the golf club is parallel to the target line at the top of your back swing, it is not across the line, it is obviously parallel. Being parallel to the target line with the shaft of the golf club at the top of your back swing is the position that you really want to work on achieving.

Across the line is a reference to the position that the golf club is in at the top of your back swing and does not refer to the clubs action through impact. If the club is swinging across the target line through impact, it would be known as either out to in, or in to out.