Side plank on the strap, dip the hip, drive it back up.
Move only at the hip and keep your shoulders stacked, no twisting the torso.
The hip drop takes a side plank and gives it a rep scheme, your bottom hip dipping toward the floor and driving back up while the strap holds the free side of the equation. All the movement comes from the lateral core, the obliques lengthening under control on the way down and contracting hard to lift your entire torso line back up. It earns a place because static side planks stop progressing quickly, and this turns the same position into loaded, repeatable side-bending work, one side at a time.
It is the progression. A side plank only asks you to hold a position, the hip drop makes your obliques repeatedly lower and lift your bodyweight through range, which builds strength a static hold cannot.
Along the side of your trunk closest to the floor. If your shoulder aches instead, restack it directly over your forearm and shorten the range of the dip.
Lower until just before your form breaks, then drive back to a straight diagonal. Range grows with strength, twisting to steal extra depth does not count.
