The performer is completely free to express, think, and create, displaying their skill, but also, in the best cases, their soul.
Jazz harnesses expression to the greatest degree via the intrinsic element of improvisation that puts musicians in the somewhat vulnerable position of just letting the music flow out of them second by second, feeding off the musical knowledge, thoughts and emotions inside of them and those around them. The performer is completely free to express, think, and create, displaying their skill, but also, in the best cases, their soul.