Here is the first quatrain of a sonnet by John Donne, which demonstrates how he uses a number of metrical variations strategically:
Bat- ter | my heart | three- per- | soned God, | for you |
as yet | but knock, | breathe, shine | and seek | to mend. |
That I | may rise | and stand | o'er throw | me and bend |
Your force | to break, | blow, burn | and make | me new. |
Donne uses an inversion (DUM da instead of da DUM) in the first foot of the first line to stress the key verb, "batter", and then sets up a clear iambic pattern with the rest of the line (da DUM da DUM da DUM da DUM). In the second and fourth lines he uses spondees in the third foot to slow down the rhythm as he lists monosyllabic verbs. The parallel rhythm and grammar of these lines highlights the comparison Donne sets up between what God does to him "as yet" ("knock, breathe, shine and seek to mend"), and what he asks God to do ("break, blow, burn and make me new"). Donne also uses enjambment between lines three and four to speed up the flow as he builds to his desire to be made new. To further the quickening effect of the enjambment, Donne puts an extra syllable in the final foot of the line (this can be read as an anapest (dada DUM) or as an elision).
Rules for poetry are important but to a certain extent you have to follow the Duke Ellington rule: "if it sounds good, it IS good."