First of all, one must understand the conception that, just as a flower can be observed blooming in nature, the Flower can be used as well as a metaphor for all things in the Noh.
When speaking of flowers, in all their myriad varieties, it can be said that they will bloom at their appointed time during the four seasons; and because they always seem fresh and novel when they bloom at that appointed season, they are highly appreciated. Flower, charm and novelty: all three of these partake of the same essence. In performing sarugaku as well, when this art appears novel to the spectators, they will be moved to find it attractive. Flower, charm and novelty: all three of them partake of the same essence. There is no flower that remains, and whose petals do not scatter. Yet it is just because the petals scatter that, when the flower blooms again, it will seem fresh and novel.
However, a note of caution is necessary. When one speaks of novelty the term does not necessarily refer to some means of artistic expression that never existed before.
Over and above this it is important to know that a Flower blooms by maintaining secrecy. It is said “When there are secrets, the Flower exists, but without secrets the Flower does not exist.” Understanding this distinction is the most crucial aspect of the Flower. The Flower of the actor is possible precisely because the audience does not know where the Flower may be located. The spectators merely know they are seeing something skillfully performed but they cannot recognize the Flower as such.
Thus the technique can represent the Flower of the actor. The Flower provides the means to give rise to a sensation of the unexpected in the hearts of the audience.
from ‘Style and the Flower‘ (1418) On the Art of the Noh Drama: The Major Treatises of Zeami, English translation J. Thomas Rimer and Yamazaki Masakuzu, Princeton U. Press, 1984.