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.
They do this in unsecured locations when one Team Member is focused on doing something "downrange"
If a threat develops you can YANK your Buddy aside.
For an OP like this they'd have a "Spotter" Filming/Monitoring them from an elevated Position, perhaps several to cover the expected route.
There were at least five undercover agents at the scene, and at least two filmed the assault.
After the assault, the man who linked arms with the man in camouflage made his own hand signals.
Those who label words as violence do so with the sole purpose of justifying violence against words.