The symbol you used in your example is indeed a hyphen, but you
are using it to function as an em dash. So technically, you're
using the wrong symbol. The hyphen ( - ) should only appear in
compound words and phrases (as in, eight-year-old, ex-wife,
stick-in-the-mud, a part-time job, etc.). The em dash ( — ) is
best explained by the Chicago Manual of Style: Em Dashes are
used to set off an amplifying or explanatory element and in that
sense can function as an alternative to parentheses (second and
third examples), commas (fourth and fifth examples), or a colon
(first example)—especially when an abrupt break in thought is
called for. - It was a revival of the most potent image in
modern democracy—the revolutionary idea. - The influence of
three impressionists—Monet, Sisley, and Degas—is obvious in her
work. - The chancellor—he had been awake half the night—came
down in an angry mood. - She outlined the strategy—a strategy
that would, she hoped, secure the peace. - My friends—that is,
my former friends—ganged u