This "exercise" was invented by Joe DeShaw one morning in the kitchen of the house when we were roommates a few years ago. I put quotes around exercise because this is not so much of an exercise for building muscular endurance or strength, but rather a mobility exercise for the shoulders and base of the neck. The Bent-Over BroomStick exercise is excellent for increasing functional capacity and dynamic posture by focusing on contracting the lower trapezius and rhomboids. The emphasis is on "Arc-Pause-Slide":
Arc: The motion of the stick out in front of the shoulders.
Pause: Make sure to pause out front to ensure that you don't round the corner and cheat.
Slide: Down the back of the head and make sure that you don't jut the chin down to get out of the way of the stick.
Each time you do this use the stick along the spine to achieve correct positioning before starting the exercise.
Have Fun!