Once you've completed all twelve of Assassin's Creed 3's main story Memory Sequences you'll unlock a new side quest called Animus Synching. This can only be played online and appears as an additional menu option on the map screen.
The objective of this side quest is to place Pivot markers on the maps and use them to triangulate the position of new Pivots. There are ten of them in total to track down, and each Pivot located unlocks a new cheat in the main menu.
Pivot markers can only be dropped at your current location on the map by going through the Animus Synching menu, and once you've placed at least three of them here they'll join up and indicate a highlighted green area that contains the next Pivot.
By moving the Pivot markers closer together or dropping in additional Pivots you can keep narrowing down the target area until it's small enough to easily explore on foot. Look out for a square icon on your minimap that indicates where the next Pivot is, then collect the unicorn-like marker at that location.
Keep checking the Animus Synching menu for progress on when the next Pivot will appear, this normally happens every 15 minutes but is speeded up if you have one or more people on your friends list who are also collecting them. By finding all ten Pivots you will have unlocked every cheat, as well as the Head in the Cloud trophy/achievement.
Any cheats unlocked can be accessed via Animus Hack at the bottom of the Options menu. No progress is recorded while cheats are active, so make sure you turn them all off before attempting to advance any further with the game.
The full list of cheats available is as follows:
- Made of Steel - You can't take any damage.
- Infinite Ammunition - You have infinite ammunition.
- Sun and Moon - Set the time to day or night.
- Killing Spree - You can always assassinate, even in combat.
- Semi-Automatic - No reload time for ranged weapons.
- Ninja - No Guard Detection or Investigation; combat/restricted areas still cause detection.
- Recruit - You have infinite Recruit Tokens to use Assassin Abilities.
- Thunder Kill - Thunder and Lightning flashes around you every time you kill an enemy.
- Weather Man - Set the weather to rain/snow or sunny.
- Season Changer - Set the season to Summer or Winter.