Three sets of cogs must be found around the map.
One is at the Spartan Monument
One is at the Intersection of Treasuries, close to the Ra Perk.
One is at the Intersection of Treasuries, close to the Stoa of the Athenians.
You must throw a spear from the shield at the cogs right as the cogs begin to touch. If the cogs are stuck on each other and both are spinning, you were successful.
When all three are done, head to the Stoa of the Athenians. Three statues should be spinning.
Time the spear to throw it at the cog when all three statues face inside.
If successful, your screen should shake.