Anapanasati, unconscious, spontaneous and natural way of breathing
Shakyamuni Buddha never taught any form of specific breathing. He was speaking about anapanasati, commenting: "When your breath is short, observe (recognise), it is short. When your breath is long and deep, observe it's long and deep".
Understand that anapanasati is unconscious, natural, spontaneous. Effortless observation.
As soon as you bring consciousness to your breathing, you will instantly modify its rythm and consistency. Even if you can obviously extract great health benefits out of conscious worked out breathing, effort will affect your consciousness and bring it far from the essential rest it is spontaneously looking for.
During zazen, the right posture, different levels of absorption can be experienced, from the conscious concentration and focus until the surrender of breath itself.
Once the breath is left to itself, anapanasati, then the real rest is natural condition... nothing is complicated.