If you are a golfer who is plagued with a slice (a shot starting left of the target curving well to the right of the target for the right-handed golfer) the club approaches above the ball.
It continues its journey to the left of the ball and is known as an out-to-in swing.
Miss the box
If the clubface is open at impact then the result will be the sliced shot. If the clubface is square when it strikes the ball with an out-to-in path we will see a pulled shot to the left, which often occurs more with the short and mid irons.
When practising, place a shoe box with the toe of the iron an inch from the box.
The iron will strike the box if your swing is presenting an out-to-in pattern.
Make some practice swings without a ball, feeling the clubhead swinging closer to the body on the downswing.
When you have missed the box three consecutive times using only an iron, replace the ball and replicate the feeling.
This is an example of coaching using minimal technical input to help make a swing change using an external focus.