At the same time the diaphragm automatically gets engaged as there is a natural need for air to support an open sound. It is a simple technique when applied and easily learned. However, it is a different approach than what is taught in a class where the technical aspects of anatomy are defined as the process of producing tone. While they may be correct in a technical sense and may help to produce a good sound, they unnecessarily complicate the whole process by forcing the singer to think of things that do not relate to the process of singing in the natural sense of speech. For choral singing we are interested in the simplest process that will lead average singers to consistent vowels and sounds that provide uniform vocal production.
Many books give a lot of detail about breathing.
They will talk about blowing out, lying down observing the abdominal muscle, panting, bending, blowing out a flame, sniffing, expanding one's chest, hissing, bouncing, pulsating, etc. While these exercises may explain in an academic sense how the breath works, they do little in combining the lungs and vocal cords and training the muscles in the art of vocal production. Many of these exercises lack purpose or usefulness in that they do not focus on the basic need to develop breath control in combination with the production of sound via an open throat. The connection of vowel formation with the need for breath support seems to escape these exercises and the singers do not get better over time. The only thing that will combine breathing and sound production are vocal exercises.