In a set of four photographs, he shows how shoulder shrugging is posturally antithetical to someone who is in control or command. He called it the direct action of the nervous system and used it to explain, for instance, trembling as an expression of fear. Saying that trembling is caused by the direct action of the nervous system may not be all that enlightening, but I don't know that anyone has yet thought up anything better. One problem that readers have had with Darwin's book is that he does not seem to have any compelling theory of why expressions should be antithetically expressed. The way to minimize the chance of that mistake is to make yourself look as unlike a dominant individual as possible.
Antithesis therefore can make sense.
But Darwin ignored communication. In the last chapter he wrote one paragraph acknowledging that expressions have communicative value, but did not mention that this was relevant to their evolution. The omission is a mystery and has never been satisfactorily explained. Ekman does have some interesting thoughts about it, inspired by historian Richard Burkhardt, and they take us back to the origin of Darwin's interest in the emotions. He was reading widely in relevant, and less relevant, material and picked up Charles Bell's Anatomy and Philosophy of Expression.