Both Uses of

flourish
in
Fahrenheit 451

Now, it plunged the book back under his arm, pressed it tight to sweating armpit, rushed out empty, with a magician's flourish!
p. 35.2 *

flourish = showy gesture

Later, the men around Montag could not say if they had really seen anything Perhaps the merest flourish of light and motion in the sky.
p. 151.2

flourish = show

Definitions:

(1) (flourish as in: the business is flourishing)
to thrive (grow or develop well)
(2) (flourish as in: dismissed them with a flourish)
a showy gesture

or:

the act of waving -- a hand or an item
(3) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

In Shakespeare's plays, flourish is often used as a stage direction indicating a specific type of showy gesture: fanfare played by trumpets or other horns to announce the entry or exit of royalty.

Check a comprehensive dictionary for other senses including some that are of interest to musicians.