One thing I've learned from many poetry workshops is that the sections of my poems that really hit it off with readers are those lines or phases which dramatically break a previously set rhythmical pattern. Like an orchestral piece of music, you take comfort in the ever-predictable musical phrases. However, it's the line that varies from that predictability that stops the show and turns out to be a crowdpleaser.
I figure it works like the architecture of a good joke. Subverting expectations creates a laugh, creates a little heart squeeze.
In the book, On Poetry & Craft, the compilation of Theodore Roethke's essays and random thoughts, in the essay called "Some Remarks on Rhythm," Roethke explores the ways rhythms work to serve our poems:
While our genius in the language may be essentially iambic, partially in the formal lyric, much of memorable or passionate speech is strongly stressed, irregular, even 'sprung.'
What about the rhythm and the motion of the poem as a whole? Are there ways of sustaining it, you may ask? We must keep in mind that rhythm is the entire movement, the flow, the recurrence of stress and unstress that is related to the rhythms of the blood, the rhythms of nature. It involves certainly stress, time, pitch, the texture of the words, the total meaning of the poem. We've been told that a rhythm is invariably produced by playing against an established pattern....It's what Blake called "the bounding line," the nervousness, the tension, the energy of the whole poem. And that is a clue to everything. Rhythm gives us the very psychic energy of the speaker...
It's nonsense, of course, to think that memorableness in poetry comes solely from rhetorical devices, or the following of certain sound patterns, or contrapuntal rhythmical effects. We all know that poetry is shot throughout with appeals to the unconsciousness, to the fears and desires that go far back into childhood..."