One of the most mistaken concepts in the XXIst century is the concept of leggerezza. It is mistaken with an inconsistency and not appoggiato sound or an undersinging attitude.
In fact is one of the fundamentals of singing and foundation of power and strength, also of touching pianissimo. It is simply the elasticity in vocal tract and especially in the larynx to follow the increasing pressure of breathing without opposing a force to maintain a fix position. In the beginning it is a basic to shape correctly the breath (that is not the flow of air but the stretch and pushing of a muscle and a capacity of getting a specific internal posture). In more advanced stadiums it is basic to have a correct height of larynx and registers passages, not to mention all the nuances a voice can have. It is also indispensable to shape a strong muscular base without straining the vocal folds in voices that will be strong and with a good ring and rock quality.