[BerntR]

Air,

The hip slide will add steepness to the motion. When you move your hips forward, hold your shoulder turn and keep your head behind, the right shoulder will drop. This motion will contribute to a steeper swing plane. So more hip slide is required for a steeper swing plane. Or a steeper swing plane is required for more hip slide. Whichever way you choose to see it.

The flatter you swing, the more the swing plane will coincide with the core rotation. So less need for added steepness and hip slide.