The base position of stable belly flight: hips forward, chest to earth, head up, arms and legs slightly spread. It stabilises you without you balancing at all.
The arch is why stable freefall isn't a balancing act. The position shifts the centre of drag ahead of your centre of mass — the same principle that makes a shuttlecock always fly cork-first. The shape aligns you; your muscles don't. The classic error is the de-arch under stress: giving up the hollow back, pulling the shoulders up, tucking the legs — and you are on your back or turning away. Which is why the sport's mantra isn't just 'arch' but 'arch and relax': tensing up flattens the shape rather than steadying it, and asymmetry turns you. Worth being clear about: this entry explains what the position does and why. You learn it in ground school and in the wind tunnel, with a coach and immediate feedback — never from a text.