When one muscle in a system is involuntary, then the entire system should be treated as involuntary. There are muscles involved in breathing that we can volitionally control, but there are other muscles like the diaphragm that cannot be controlled due to them being involuntary. Thus, the entire breathing system should be treated as involuntary. No manual control, but a focus on setting up automatic activity.
What does this mean for the singer? Well, it means that we do not have to divert energy to flexing muscles in our abdomen or back or rib-cage intercostals. All that accomplishes is unnecessary breath expenditure in the form of energy waste, as well as excessive muscle contraction which turns into rigidity.
Remember, we are all about firm yet flexible. If our registration is functioning well, then the moving air will be regulated efficiently by the size of the glottal slit, and neither breath nor energy will be wasted. The muscles of breathing will expand and contract as needed given the correct expenditure determined at the sound source. This is an automatic response with precise calculations done by our autonomic nervous system so that we do not have to fret about our breathing.
Just think, we walk around and our body takes care of our breathing all day every day with us speaking and singing spontaneously, without us “doing” anything. Know cause, work on cause, let the rest respond automatically.