Expansive only in explosions

From Willa Cather's, "Song of a Lark":

Ottenberg was watching her brilliant eyes and her face. She was much
handsomer than she had been early in the evening. Excitement of this
sort enriched her. It was only under such excitement, he reflected,
that she was entirely illuminated, or wholly present. At other times
there was something a little cold and empty, like a big room with no
people in it. Even in her most genial moods there was a shadow of
restlessness, as if she were waiting for something and were exercising
the virtue of patience. During dinner she had been as kind as she knew
how to be, to him and to Archie, and had given them as much of herself
as she could. But, clearly, she knew only one way of being really
kind, from the core of her heart out; and there was but one way in
which she could give herself to people largely and gladly,
spontaneously. Even as a girl she had been at her best in vigorous
effort, he remembered; physical effort, when there was no other kind
at hand. She could be expansive only in explosions. Old Nathanmeyer
had seen it. In ther very first song Fred had ever heard her sing, she
had unconsciously declared it.

p. 373