In the mission “Loose Cannon” in Assassin’s Creed: Brotherhood, you are tasked with destroying one of the war machines created by Leonardo Da Vinci for the Borgias. During one section, you must escort a man on a boat through a guarded tunnel. To achieve full synchronization, you’re instructed to “Open 1 of the first 3 gates,” but if you try to open the gates, you’ll find that you can’t interact with the switch.
So what do you do? The problem is that the instruction is unclear, or rather, incomplete. It should read “open 1 of the first 3 gates before the guard does.”
What you have to do is run ahead of the boat using the rafters (there’s only one path, so you can’t get lost) and assassinate the guard manning the gate switch. Detection will causes instant desynchronization, so use your Hidden Blade. Then open the gate yourself. You’ll get full synchronization at the end of the section.