The manganese cluster of photosystem II is used to store charge, as it is easily able to be oxidised and reduced, through the S-state cycle.
During charge separation on photosystem II, an electron hole is formed on the chlorophyll by an electron migrating up photosystem II. An electron can be provided by the manganese cluster, allowing the hole to be refilled. This puts a positive charge on the manganese cluster. This leads to the special pair chlorophyll being excited, allowing another electron moves to plastoquinone B. The positive charge on plastoquinone allows 2 protons to be taken up from the stroma, moving protons for the proton gradient. Another electron is donated to the special pair chlorophyll from the manganese cluster.
To restore the positive charge on the manganese cluster, water molecules are oxidised. By producing O2 molecules (O=O) the Mn cluster returns to its original state, demonstrating its catalyst activity.