The broker derives the cash quote from the underlying futures, adjusted for the time to expiry, and adds its spread. The instrument then trades on a value per point basis, so a position is described as so much per point rather than in lots.
Two features distinguish it from a futures position. There is no expiry on a cash index CFD; it rolls indefinitely and pays the overnight-financing-charge instead of embedding it in a forward price. And the minimum size is far smaller, which is the main reason retail traders use it.
Index CFDs also trade outside the underlying cash market's hours, using the futures as reference. Liquidity and spreads outside those hours are materially worse, and gaps at the cash open are common.
Example: GBP 3 per point on an index at 18,420 is GBP 55,260 of exposure. At 5% margin under the 20:1 major-index cap that is GBP 2,763, and a 60-point move is GBP 180.
Related: cfd, overnight-financing-charge, dividend-adjustment, spread-betting