When reading, you could go back at the sentence and try to remember if you know one of the words in the phrase or sentence. When listening to someone saying this sentence, You won’t have time to figure out what any of the words means. You can’t tell that person: “wait!” and then open your dictionary and ask him: “how do you spell ‘enthrall’?”
Also, unless it’s an informal situation (talking to a friend.), you wouldn’t be able to ask him/her : “what does “repartee” mean?”.
So, the question is: how is it possible to understand something when you hear it for the first time and you can’t even check how it’s written or what the words in it mean?”