“What do you mean?” I say, my brows furrowing in confusion. - Is this a participial phrase?

Question

I can't understand how the phrase "my brows furrowing in confusion" doesn't have any verb. In the Elements of Style, I read that "A participial phrase at the beginning of a sentence must refer to the grammatical subject." I wonder if the sentence below provides a participial phrase or something else.

“What do you mean?” I say, my brows furrowing in confusion.

Answer 1

That is a participle clause, but it doesn't "attach" to anything because it has its own subject, my brow. Such loosely attached clauses are called "absolutes"; see this question and this one.