In the left photo, we can see that at address, this golfer’s spine is approximately vertical, perpendicular to the ground.

In the right photo, we see that as he reaches the top of his backswing, his spine has tilted left approximately 35 degrees.

We are watching the swing from two different angles to measure the same thing because as the golfer’s hips and shoulder complex turn, the golfer’s orientation moves from the back facing the left rough to the back facing the target.

This change in spine angle is a side tilt to the left, combined with extension and turn it allows the golfer to keep the head centred.

Keeping the shoulder centre stable enables the club to return to the ball in a consistent manner (low point control).