They do not have a filtration barrier in which to collect the contaminate but instead use a magnetic field to trap the passing ferrous particles within the filter body. Because they do not have a mesh, they are not restricted to a minimum particle size. Therefore, they can catch sub-micron particles that would escape through a conventional filter.
The downside to this type of filter is that it can only hold ferrous particles that are attracted by the magnetic field. Other contamination such as rubber wear & silicate particles will pass straight through the filter so it is often advisable to fit another hydraulic filter in the system to supplement the Magnom product.