The Reference symbols are cyclically shifted by the PCI Mod 6.
So if PCI Mod 6 = 0, the first RS will be found at the "edge" of the resource grid.
If PCI Mod 6 = 1, the first RS will be found one RS into the grid.
And so on until 5.
There are 12 RE per RS, so this guarantees that you maintain 2 RS per RE.
This is done so RS from adjacent cells stomp on each other as little as possible.
Hope that helps.