App note from Littlefuse on how do transient suppression devices protect electronic circuits. Link here (PDF)
There are two major categories of transient suppressors: a) those that attenuate transients, thus preventing their propagation into the sensitive circuit; and b) those that divert transients away from sensitive loads and so limit the residual voltages.
Attenuating a transient, that is, keeping it from propagating away from its source or keeping it from impinging on a sensitive loadis accomplished with filters inserted in series within a circuit. The filter, generally of the low-pass type, attenuates the transient (highfrequency) and allows the signal or power flow (low-frequency) to continue undisturbed.
Diverting a transient can be accomplished with a voltage-clamping type device or with a “crowbar” type device. The designs of these two types, as well as their operation and application, are different enough to warrant a brief discussion of each in general terms.