The man in the camouflage on the right is wearing noise-canceling earphones.
He's also filming.
So the hand signal establishes a chain of custody for the evidence.
The feds can say, "Here's Agent Smith giving the prearranged signal that Antifa is following through on its threat against Andy Ngo."
Lawyers for Portland won't be able to argue that this person is not Andy Ngo.
Those who label words as violence do so with the sole purpose of justifying violence against words.