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.

It's a doing or action word. For example… The verb in that sentence is 'ate'. An adverb is a word that describes the verb. The adverb in this sentence is 'quickly'. It tells us how Zak ate his ...