Williams's distinctive use of poetic form, with short lines and lots of enjambment, results in a poem that stretches out time like a rubber band. Specifically, time moves in an arc, going faster, then slower, then faster again. These effects are extremely subtle, and we're talking about a difference in line lengths of only a few syllables. The poem captures a few moments as if inside a frame, so that we can scrutinize every detail. It demonstrates that the perception of time is relative to the mood and position of the observer. Charles Demuth's famous painting (see it here), inspired by the poem, provides one artist's take on how the experience with the fire truck is broken up into distinct moments.
Time is divided into smaller moments in the middle of the poem, as the fire truck passes closest to the speaker.
The poem can be recited without any pauses without distorting its meaning.