The phrase "hip bones" refers to the two bones located in your hips that connect your legs to your torso. They are sometimes used as a reference to a person's body shape or figure. Full definition
For your waist, wrap a measuring tape across your back, just above the highest point of hip bone or 1/2 way between here and the bottom of your 10th rib. (ultrawellnesscenter.com)
Apparently this also helps with proper breathing — i.e. keeping ribs aligned with hip bones. (femfusionfitness.com)