Reverse spine angle is not to be confused with a reverse pivot, which looks similar but is a problem of poor weight shift.
Maintain some forward flex
Reverse spine angle is not to be confused with a reverse pivot, which looks similar but is a problem of poor weight shift. Reverse spine angle occurs when weak oblique and glute muscles force you to lose all of your flexion as you swing to the top. If you are bent at the hips 30 degrees at address, but that shifts to minus-5 degrees at the top, you have reversed your spine angle. Your torso will lean toward the target. This is a major cause of lower-back pain and poor contact. To start breaking 90, maintain some forward flexion from address.
Drop left foot back to check your angle
This is a somewhat cruel but useful drill to check if you swing into a reverse spine angle at the top. At address, drop your left foot back almost behind your right. Now swing to the top. If you begin to fall over, you've decreased the angle that you bent forward at the hips way too far. Focus on keeping some flexion as you swing to the top, and you won't lose your balance.
Kneel and turn to train the obliques
Using a narrow stance, get into a half-kneeling position and take a wide grip on a club with your arms extended. Keeping the lower body still, rotate the torso toward the bent knee as the arms move up and across the body. The rotation will strengthen the obliques, and the kneeling helps the glutes. Do a set of 10 in each direction, switching the leg positions.