The goal is to get a consistent cut of sand from your angle of approach and the low point of your swing.
Mike Malaska draws a line in the sand with his sand wedge and sets up with it just forward of his stance. The goal of this exercise is to make swings into the sand and take the line out. Mike aims for the line, and the sand disperses in front of where the line used to be.
This is a great exercise to figure out where to place the ball. The low point of the swing is about 2 inches in front of where the line was. The low point in your swing is where you want the ball to be. This gives you the best chance to get the sand and throw the ball out. Practice this, allowing you to be consistent with where you hit the sand behind the ball.
Mike then draws another line in the sand and places two balls in front of the line. He makes practice shots swinging down the line before he gets to the balls. Then Mike hits the line behind the balls and gets the ball out of the bunker. Try not to anticipate how the club hits the sand but focus on hitting and taking the sand.
You must be good at how the club bottoms out. This will help you be more precise with your bunker shots.
Practicing hitting a line in the sand gives you confidence, and it is a great skill to understand when you set up where the club hits the sand. This principle also applies to full swing on the fairway.
Again, the drill's purpose is to draw a line in the sand, swing, and take the line out. You want to put the ball at the low point of the swing.