Like an iamb, a trochee, and a spondee, a pyrrhus, too, is a type of foot in a metrical feet. While a spondee is a pair of stressed syllables, a pyrrhus, is a pair of unstressed syllables, where one unstressed syllable is followed by another unstressed syllable. And much like a spondee, a pyrrhus is also used as a substitution within a metrical template to call attention to something the poet deems important.
With a bare bodkin? Who would fardels bear?
With a | bare bod | kin who | would far | dels bear
In this example, a line from one of Shakespeare's popular soliloquies (To be or not to be...) in Hamlet, there are two substitutions, a pyrrhus followed by a spondee. When poets break a metrical pattern it is purposeful; the break informs the audience (in the case of a play) and/or the reader of a shift in emotion or mood in the play or poem.