Lie on your back with your shins resting on a box or bench so your hips and knees each sit at a right angle. Exhale slowly and fully, pushing all the air out until you feel your ribs drop and your lower back flatten into the floor — that flattening is the position you're after, not a lift of the hips. Breathe in through your nose without letting your ribs flare up or your back arch away from the floor, then repeat the long exhale. The 90/90 leg position takes your hip flexors out of the equation so the effort shows up in your ribs and diaphragm instead.