They are made up of two non-floating pieces and therefore lighter than single blocks;

the ergal spider anchored to the track reduces the vibrations transmitted to the pads / pistons;
the grooves and fins on the braking track dissipate the heat facilitating the cooling of the disc.

To achieve such performance these discs receive extraordinary attention for the MTB sector in the production phase.
To go into the details I asked directly to those who produce them, that is to Cesare Andrea Brioschi, owner of BCA, who not a wealth of hyper-technical details explained to me the following.

Before that, however, I would like to point out that BCA has been operating for over 20 years in the motorcycle brake sector as a supplier of first assembly for brands such as Fantic, Bimota and many others. The spider (whose design is specific for each diameter), made in Ergal, with a particular anodizing to obtain the finish you see in the photos.
Its structure is designed to give stiffness and stability to the braking surface.

The construction of the disc is of the non-floating type (contrary to what was written at the time of publication of the article, NDR) and the braking surface in 2.15 mm thick AISI 420 steel is the real highlight.
The treatment it undergoes at a thermal level serves to make this steel better compatible with the brake pads most commonly used in the MTB field.
The braking track then receives special millings that facilitate cooling in the upper part and the upper fins that do not come into contact with the pad are used to cool the disc.
The grooves we see on the disc are cut with a laser and the subsequent mechanical processes are divided for each face of the disc. This increases production times and consequently costs.

All these precautions are used to optimize the wear of the pad which, as confirmed by our test, is not as high as one would expect, because the braking surface is able to keep the surface patina of the pad “clean”.
The wavy design, in fact, allows the pad friction material to be "cleaned" while the brake is running and the grooves on the braking track play the same role but with very different methods and efficiency compared to other discs.

In essence, this device allows the discs to optimize friction with the pad without consuming it prematurely, but reducing the overheating of the braking surface.

And this is a crucial fact for the optimal functioning of any brake:

The lower the temperature, the longer the brake can be efficient.
The lower the temperature, the lower the risk of pad vitrification, that is the phenomenon that drastically (and sometimes dramatically) reduces braking power.