A bad backswing is often due to one of three things: a sway (you lose power), a reverse pivot (you slice or pull) or an unflexed right leg (you hit it thin).
A shaft without a head on it can help you correct all three faults. To determine which problem is affecting your backswing and then correct it, try these three drills.
Place the shaft in the ground behind and between your legs, angled so that the handle of the shaft touches your inner right thigh. Now take your backswing. If your right leg pushes the shaft forward toward your front leg, you're reverse pivoting. Practice until you can turn without moving the shaft.
Push the shaft into the ground outside your right leg, angled so that it touches your foot and leg up to your knee. Now take your normal backswing. If your right leg pushes the shaft back, you're swaying. Practice until you can turn without moving the shaft.
Place the shaft in the ground behind you so that the butt end of it presses against the back of your right knee. Now take your backswing. If your right leg pushes the shaft upward into the back of your thigh, you're not flexing your right leg enough. Practice until you can keep the club behind your bent right knee as you swing.