If your power line ever surges above the clamping voltage, the internal "clamping device" will rapidly lower it's resistance from virtually an "open circuit" across the power line to virtually a "short circuit" across the power line. Depending on the magnitude and duration of the surge, this will either cause the surge to be harmlessly absorbed, or it will blow the hell out of the "clamping device," leaving your power strip and the devices it powers unprotected from surges. Some power strips incorporate circuitry that will warn you if this happened, but most do not. Testing by the end user is usually not feasible. Most "clamping devices" are metal-oxide varistors (MOVs) rated by (1) their clamping voltage and (2) the maximum energy they can absorb without catastrophic failure.