Whether a quality is perceived as being false or legitimate is dependent upon the way in which the configuration of the glottis is regulated by tension assumed by the arytenoid system. When the response of the arytenoids is of such a kind that they are incapable of thickening the vocal folds as intensity is increased, the tonal product will express itself in terms of a variety of different types of false tone qualities.
The essential mechanical difference among the numerous types of falsetto is that each reflects a substantial difference in the balance of tension distributed between the openers and the closers of the vocal folds. As cricothyroid (stretchers) tension is countered exclusively by the posterior cricoarytenoids (openers) the falsetto will be pure. When tension shifts to the lateral cricoarytenoids (closers), the result will be an occlusion of the vocal folds, the falsetto will gradually lose its falseness as the tone quality becomes more full-bodied and complete.
Cornelius L. Reid 