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.