In these examples, down and back are not prepositions but function as adverbs to extend or change the meaning of the verb. This combination of verb and adverb is always known as a phrasal verb.

A phrasal verb is a verb that is combined with a preposition (e.g. in, on, with) or an adverbial particle (e.g. up, out, off) The preposition or adverbial particle extend the meaning of the verb ...