A manner adverbial or adverbial of manner is an adverbial that modifies with respect to manner.
As a single-word adverb, these are adverbs of manner, e.g. quickly, quietly, etc.
We can also use prepositional phrase adverbials with "like" or "in", i.e. a prepositional phrase.
With the preposition "in", followed by a determiner, an adjective, and a noun particularly "way", "manner" or "fashion", i.e. "in [determiner] [adjective] [noun]". E.g: "she got pregnant in the usual way."
With the preposition "like", the manner adverbial is used to express simile, e.g. "she slept like a baby". These are often used with copular verbs, e.g. "It smelt like fresh laundry."
With the preposition with, e.g. "with a lot of difficulty"
A present participle clause acts as a manner adverbial.