What it is: That spine-wrenching position at the top of your swing in which your head tilts toward the target (usually because it's locked on the ball), your right hip juts out away from the target, and most of your weight is on your left side.
How to know if you're doing it: You hit a random assortment of fat and thin shots.
Using your left fingers, lift up the butt of the club so that it is under your chin and rests on your sternum. The club should hang straight down from there, so that the toe of the club is pointed at the ball. In this position, the clubshaft represents your spine angle at address.
Still holding the club, make your normal backswing, taking your right hand back until your fingers are pointing at the sky.
If you are pivoting correctly -- that is, if your spine angle has remained consistent during the backswing -- the club hanging from your left fingers will continue to point down at the ball.
If you're reverse pivoting, however, your incorrect tilt will force the clubhead to swing to the right, past your right knee.
Groove your backswing to the point that when you do this drill the club never moves more than a couple of inches to the right of the ball.