The whole body sings. Many people, while chanting or singing, do not engage the muscles of the core, limiting their vocal power and resonance. Voice scientists have found that during long periods of singing, muscular fatigue occurs in the lumbar-thoracic region of the back, especially in the latissimus dorsi. According to voice scientist D. Ralph Appelman, the latissimus dorsi, together with the abdominal muscles, create a sphincter that controls expiration. The latissimus dorsi are the only large muscles of the back able to do this. It is explains why many singers feel muscle engagement in their backs when sustaining long phrases of music, and many singers have the sensation of controlling exhalation from the whole circumference of the waist, including the back. If the core of the body is engaged the voice is supported.
EXERCISE: A great exercise to engage the core while singing is to sing with your back against a wall. Put your back against a wall. Now slide your back down the wall until your knees are bent and it looks like you are sitting in an imaginary chair. Press your back into the wall, take a breath and sound on a loud ahhhh. The region you want to especially focus on pressing into the wall is the lower third of the ribcage and waist area. You will find that in this position, you can get very loud and go much longer on your breath than usual. (Excerpt form Vocal Yoga, the Joy of Breathing, Singing and Sounding.)
I like this exercise but haven’t managed to reproduce the effect when I’m away from the wall. How do I make the transition?
When you are against the wall you are activating the abdominal muscles and the deepest layer, the transverse. I would try to explore this muscles by pulling your abs in gently when you make sound. Make sure to release the abs out for breath and then pull your naval toward your spine as you make sound. Hissing on an sssss will give you the feeling of the activation of the abdominal muscles, which is called support. Also, try singing in the yoga pose Navasana.