Crescendos build anticipation and emotion. Ravel’s Boléro is one long crescendo lasting about 15 minutes. In film and trailers, crescendos drive scenes toward a climax; in pop, the pre-chorus often crescendos into the chorus.
A swell is a short crescendo on a single note or chord, common in strings and pads.
Using it in a prompt
Ask for "a slow crescendo from 0:20 to 1:00" or "string swells" to shape emotional peaks.