Start on the same plane you'll impact on
Mo Norman used a single-plane swing: he started on the same plane at address that he would be on at impact. As explained here, matching the address plane to the impact plane simplifies the motion and allows you to be more consistent. The logic offered is to start as close to impact as you can at address so you can get there at impact.
Channel: Todd Graves
Watch official source at 0:56Why this context matters
Explained via Mo Norman's swing in a short promotional clip.