Final answer:
The phrase 'his heart pounding with the effort' is a participial phrase that adds vivid detail to the subject's actions in the sentence by describing the state of his heart.
Step-by-step explanation:
In the sentence "He ran up and down the stairs, his heart pounding with the effort," the phrase "his heart pounding with the effort" is a participial phrase. A participial phrase includes a participle and operates adverbially to modify a verb, an adjective, or a whole clause. In this case, it provides additional detail about the subject's action by describing the state of his heart as he moves. The inclusion of such a phrase enhances the sentence by offering a more vivid description of the activity and its effects on the subject.
Participial phrases are valuable tools in writing, as they can convey an array of actions and sensations, contributing to the rhythm of the prose and allowing readers to experience a simile, exemplified when one's heart is described as "beating like a hammer." This imagery and descriptive detail propel the narrative just as the list of verbs does in the provided text examples.