A useful analogy makes this clearer. Consider a water mill protected by a pressure relief valve. The pressure relief valve does nothing until an over-pressure pulse occurs in the water supply. When that happens, the valve opens and shunts the extra pressure aside, so that it won't reach the water wheel.

If the relief valve was not present, excessive pressure could damage the water wheel, or perhaps the linkage for the saw. Even though the relief valve is in place and working properly, some remnant of the pressure pulse will still reach the wheel. But pressure will have been reduced enough not to damage the water wheel or disrupt its operation. This describes the action of surge protective devices. They reduce surges to levels that will not damage or disrupt the operation of sensitive electronic equipment.

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