The diagram shows the cross-section of the curved area. There is an intersection of the esophagus and the lower pharynx. Voice is produced around the part where the pharinx is narrowed. The actual vocalization is made by a kind of friction caused by the vibration of the mucous membrane inside the esophagus by the air pushed onwards by the pressure in the abdomen.
So air is taken into the lower part of the narrowed pharynx. If it is taken deeper than there, a portion of the air would remain in the esophagus every time the vocalization is made. Eventually the accumulated air would fill the whole intestines and blow out the abdomen. It is really embarrassing and sometimes causes a clinical problem.
