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A guard ring is a ring (or very nearly so) surrounding a sensitive part of a circuit and connected to earth or a low impedance voltage which is at the same potential as the sensitive bit. It is so leakage currents through the air or across the printed circuit board flow into the guard ring and not into the sensitive circuit. Only really used with voltages in excess of 500V or impedances greater then 100 M ohm.
Now, how they effect latch up is a mystery, though its good to keep leakage paths under control. There are many sort of latch ups, I suppose you mean semiconductor devices when driven with too many volts, suffer an avalanche breakdown, which if the current is limited is non-catastrophic, i.e. when the voltage is removed the device recovers. Hence the requirement to control your high voltage leakage paths with guard rings.