Down the line
Once again, I stress there are no straight lines in a golf swing. The golf ball is on the ground and to the side of you, which means the club head swings on an arc.
In the down swing the club swings from inside the target line to on the target line at impact and then back to the inside of the target line on the through swing. To help you feel this, make a baseball swing with the club about knee high at address. Make swings and feel how the club swings left after impact.