What is series mode surge protection?
Series mode surge protectors use a large, carefully designed inductor in the hot or live circuit and other circuitry to roll off high frequency / high rate of change transients. The technology was developed and patented by Jack Hartford of ZeroSurge and is licensed by a small number of other companies who know what they are doing including Torus Power and SurgeX.
Series mode devices to not rely on the ground for surge protection and do not contaminate the ground. They are also intrinsically reliable which allows companies like SurgeX to offer a lifetime warranty on their products. They are the only point of use surge protection technology you should be using close to your AV equipment.
The diagram below from SurgeX shows how series mode technology works.