How breathing moves your shot
As you breathe, your diaphragm and chest move — and through your body's contact with the rifle, the muzzle rises and falls. Through a scope at distance this movement is dramatic; the reticle visibly floats up and down with each breath.
You cannot hold your breath rigidly and stay still — that spikes your heart rate and starves your eye of oxygen, making the reticle pulse and your vision degrade within seconds.
The respiratory pause
There is a natural 2–4 second pause at the bottom of your exhale, before the urge to inhale. The body is at its most still here. This is the 'respiratory pause' and it is when you break the shot.
The cycle: breathe normally to settle. Take a slightly deeper breath, let about 70–80% of it out, and let your body settle into that natural pause. The reticle will sink and steady. Break the shot in that window.
If you don't break the shot within ~4 seconds, do NOT force it. Take another breath and reset. A forced shot at the end of an over-long hold is a flyer waiting to happen.