From left to right, the outer piece that the outer tire bead seats on. It's the shiny part when looking at the wheel in normal fashion. Next is the wheel center, with indexing flanges for the inner and outer sections, looking like a wide band. When the bolts are tightened, the outer tire bead gets clamped between this and the outer piece. Those holes allow air to flow freely to the valve stem and (if so equipped) tire-pressure monitor. Lastly, there's the inner section which the bolts thread into, and the wheel studs go through. Since there is no need for a regular drop center, the shape of the inner section lets a deflated tire fold in, but not get out of shape or come off the rim.