Smoothness (sm) option refers to vocal fold vibrations. The vocal folds meet at the midline, as they do in normal voicing, but they do not slam together forcefully to create a very sudden cessation of airflow.
DECtalk Software uses a variable-cutoff, gradual low-pass filter to model changes to smoothness. The range of sm is from 0% (least smooth and most brilliant) to 100% (most smooth and least brilliant). The voicing source spectrum is tilted so that energy at higher frequencies is attenuated by as much as 30 dB when smoothness is set to the maximum but is not attenuated at all when smoothness is set to 0.
Professional singing voices that are trained to sing above an orchestra are usually brilliant, while anyone who talks softly becomes breathy and smooth. To synthesize a breathy voice, having the sm option set to 50 or more is good. Changes to smoothness do not have a great effect on perceived voice quality.