The breath leads the movement
Prana Vashya — Hold roughly 5 breaths, then rest 4; muscles keep a memory of the stretch, so repetition with recovery deepens range. Inhalation works on the body's structure; exhalation works on the muscles.
Anatomically: — Inhaling into a backbend is standard, but for belly-breathers an exhalation entry frees thoracic extension. In deep backbends the chest is already "inhaled," so the skill is finding deeper support.