The hummer wheels are two piece. There is a back and a front. These two pieces are held together by either 8 bolts (bias) or 12 bolts (radial). There is a piece in the middle of these two halves that serves as the beadlock/runflat. The PVC piece that Daniel sells is just a beadlock. To install it, you take the back half of the wheel and put the tire on it. Then you put the PVC piece in, the rubber O-ring, then put the outer half on. The PVC piece "pinches" the tire between the two halves, therefore preventing the tire from coming off the bead on both sides of the wheel. A run flat is one of two styles. The first one is a magnesium piece like the PVC piece, only it has a raised metal ring that stands off of it about 5 or 6 inches. After the tire is deflated so far, it contacts this metal ring and keeps the tire from completely going flat. The other style is a rubber run flat. It is the most sought-after. It serves the same purpose as the magnesium one, but doesn't do as much damage to the inside of the tire and probably rides better too.