The Hebrew term for "enthusiasm" (zerizut) translates as "alacrity," which suggests a liveliness or keen fervor. Acting with alacrity means springing forward without hesitation or obstacle. That does not mean that you are reckless or that your action is unpremeditated, but that no time is wasted, no rationalization deflects you. Not only will alacrity lead to productivity, it unifies the action and the heart so that you and your doing become one.

The fact that one is not lazy does not mean that he has acquired enthusiasm.--Rabbi Shalom Berezovsky
Alan Morinis

Just as zeal can result from an inner burning so can it create one. That is, one who perceives a quickening of his outer movements in the performance of a commandment conditions himself to experience a flaming inner movement, through which longing and desire will continually grow. If, however, he is sluggish in the movements of his limbs, the movement of his spirit will die down and be extinguished. Experience testifies to this.
Rabbi Moshe Chaim Luzzato

Enthusiasm depends on the state of a person's heart. When someone frees his heart of all other thoughts that reside in it and seizes upon one thought, then he will without doubt be enthusiastic in its execution.
Orchot Tzaddikim

Sloth is like a small tear in a garment that will destroy the whole thing if not attended to, or like a slight leak in a water pipe that might ultimately cause the whole house to flood.
Rabbi Yechiel ben Yekutiel

"You must know," says the sixteenth-century Mussar classic Orchot Tzaddikim, "that the trait of zeal is the foundation of all traits." And later it adds, "The trait of zeal is an ornament to all the other traits and it perfects all of them." If you are going to be generous, how much better to give with enthusiasm. Acts of loving-kindness should not be delayed, but rather done with alacrity. Be zealous in defense of the honor of your friend. These are examples of some traits among many that gain an important dimension when they are enlivened with enthusiasm.
Alan Morinis

It is reported that when Rabbi Simcha Zissel Ziv, the founder of Kelm Mussar, awoke in the morning, he would immediately spring out of his bed in great haste, as if a highwayman was standing behind him threatening to kill him--in order to overcome laziness and implant in himself the trait of enthusiasm.
Author Unknown