These devices are basically autotransformers. So they reduce the noise by providing inductive filtering. But they don't really protect against strong surges by themselves.

So Tripp Lite uses a regular varistor for that, just like any other surge protector. In Europe you'd be far better off buying a voltage relay and adding it to your electrical panel, but it's not usually possible with the non-modular US electrical panels.

The simple line-conditioners were surprisingly effective, and are a fraction of the cost of lab/medical grade galvanic isolation ferroresonant transformers. =3