Lighter registration is in some respects comparable to softer singing. When we sing loud, more of the vocal cord meets for the given frequency as the glottis closes. Since this results in a longer closure phase, more breath pressure is necessary under the folds to move them apart for the next vibratory cycle. Full glottal closure with increased breath pressure encourages vibrato, softer singing assists in simple tone singing.
When using a breathy tone, singers must breathe more frequently, without interrupting the musical phrase. To prevent fatigue they need to use little bits of vibrato in places where it will not adversely affect the ensemble. The most important instruction a singer needs is to stop and rest if the voice is beginning to tire while singing in this style. Finally, consider alternating musical styles in rehearsal rather than rehearsing all simple tone music at once.
Jura Litchfield says