This drill teaches you to make a shallow cut through the sand with your club rather than swinging too steeply into the sand, digging too deeply and leaving the ball in the bunker.
Making a shallow cut through the sand will get the ball out more consistently with better control.
The Tee Drill
Place your ball on a tee in the sand, and then push the tee into the sand so that the head of the tee is barely above the surface. Position the ball off your left heel. Try to clip the head off the tee when you swing.
To do this you will have to hit behind the ball with a shallow angle of approach, “slipping” the clubhead under the ball. Use very little wrist action, and focus on sweeping the club back and through.
By varying how much you open the face of the club and the length of your backswing and follow through, you can control your distance of the shot.