I came across the following sentence in a book:
She could barely keep up with the gas station attendant's directions, he jabbered so.
According to the book, the sentence means that the woman couldn't keep up with the man's directions that he jabbered.
What I do not understand is why the word so is present there. What kind of construct is used in this sentence? Is this construct a formal one?
I would be grateful if you could help me with this question.