Diaeresis
Pronouncing in two syllables a vowel sequence that makes one in everyday speech: "li-on" instead of "lion."
Diaeresis splits into two syllables contiguous vowels that everyday speech merges into one: "nu-age," "vi-olence," "passi-on" in French verse. In classical French metrics it isn't a whim but a largely etymological rule: words whose vowels were separate in Latin call for diaeresis in poetry.
For the practitioner it's first a counting tool: "violon" is two syllables in prose, often three in classical verse. It's also an expressive tool — diaeresis stretches the word, slows it, showcases it; poets use it to make a "vi-o-lence" last. Its inverse is synaeresis, merging two vowels into a single syllable.
Example
English meter knows the same play: "fire" as one syllable or two, at the line's need.