Start from the healthy sitting position with your buttocks well back into the seat angle but not leaning on the backrest at first. Then lean your upper torso straight back. When your upper back contacts the backrest, let your lumbar area relax. This approach preserves a slight degree of lumbar lordosis. You’ll know when you’ve done it correctly if you feel your weight being born through your sit bones when you lean back, not through your sacrum or coccyx. The pelvic diamond remains spacious. The posture should feel open through your abdomen and diaphragm.
—from The New Rules of Posture: How to Sit, Stand and Move in the Modern World by Mary Bond, Healing Arts Press (2007)