How can a trochaic substitution emphasize a word in a normally iambic line?

In prosody, a trochaic substitution occurs when a trochee (a stressed syllable followed by an unstressed one) replaces an expected iambic foot (an unstressed syllable followed by a stressed one) within a line of verse.1

Because the iambic pattern creates a rhythmic expectation of rising stress, the sudden reversal of this pattern—placing a stressed syllable where an unstressed one is expected—creates a deliberate tension between the established metrical pattern and the meaning of the words. This disruption forces the reader to pause or shift their focus, effectively highlighting the words involved in the substitution. By breaking the flow of the line, the poet draws the reader's attention to the specific emphasis or emotional weight carried by those syllables, providing a dynamic counterpoint to the otherwise regular rhythm.12