iambus: light + heavy
pyrrhicus: light + light
creticus: heavy + light + heavy
dactylus: heavy + light + light
Brevis Brevians is a tendency in early Latin, first attested in early metric poetry, to reduce the length of the second syllable of iambic words, resulting in a pyrrhic word. The same principle applies if the word is a cretic, which then mutates to a dactylus. It is also possible for it to occur on the second syllable of four-syllable words, but this is rare.
I don't have any examples in verses, because Plautine meter gives me chills, but I do have a number of single-word examples written down: ho̯mo̯, ma̯le̯, gu̯be̯rnabunt, ...
The application of Brevis Brevians is, in Plautus, not necessarily limited within one word. There are instances like e̯t i̯llorum and even, with elision, a̯ge a̯bdūce.
Brevis Brevians is quite common in Plautine drama, and most probably originates in an actual linguistic tendency. It is retained in the classical language in common words which kept their pyrrhic form, like mălĕ and bĕnĕ, where the adverbial -e is expected to be long, and in ĕgŏ (compare: ἐγω). Others, like homo, reverted back to a long final vowel even in other preclassical poets:
Samnis, | spurcu̯s ho|mō, vi|ta illa | dignu̯s lo|coque (Lucilius 150 Marx)
The pattern is, as one might intuitively expect, less common in declined nominal and verbal forms, because of the strong paradigmatic analogy with non-iambic and non-cretic forms that retain the final heavy syllable.
Sources: Nougaret 1956, Sturtevant 1940, Mester 1994, Marotta 2000, Hayes 1989