The tension on the diamond wires primarily controls the side bends of your mast. Upwind loose diamonds allow the middle of the mast to bend to leeward and the top of the mast to hook to windward. This tends to cause the boat to heel very easily in wind gusts. Having your spreaders setup correctly will also allow the diamond tension to "automatically" de-power the mainsail during gusts. This is achieved by pushing the middle of the mast slightly forward and taking up the luff curve of the sail and flattening it out.
Generally speaking, we run our diamond tension as a way of powering or de-powering your rig. Turn the tension on to de-power and turn it off to power up. Think of this as a "master cunningham".
Turning the bolt on the mast base will adjust your diamond tension; this can also be done once the boat is fully rigged. (See the tuning guides for general settings).