The Swing Click is a fantastic training aid to groove tempo in the backswing.
The device makes a clicking sound as soon as the forearm is in the correct position at the top of the backswing, at impact and finish. By practicing with a Swing Click, you can train your brain to wait for the click at the top of your backswing. The process trains you to be in the same position at the top of your backswing every time, resulting in more consistency.
I like this for the backswing. It helps you set the club at the top, which is key for better rhythm and tempo throughout the swing.