If you stand straight up with your feet together and lean to the right or left, this is considered side-bend. Side-bend is a key factor in good golf swings since you will need quite a bit of bend to make a backswing and get to impact. You must also be able to rotate while you are side-bending.
Side bend is actually when the spine moves sideways. The key to the golf swing is bending sideways while staying tilted.
At address, you must make sure you are tilted and the sideband is away from the lead hip approximately 15 to 20 degrees.
When you rotate the torso with the ideal side bend your arms move into the backswing. Then the trailing arm folds to the top of the backswing. The bend of the spine actually moves toward the lead hip as you maintain the spine tilt and rotate.
At the top of the backswing, you remain tilted and your trail arm is folded wide enough to be “away” from the body.
As the torso turns toward the target, the spine increases its tilt and the leverage angle between the arm increases as the arms move toward the ball.
Much of the way the spine works in the swing is due to how the pelvis moves. Make sure you have the proper pelvis rotation around a stable trail leg to maintain the spine tilt and sideband.