To implement Quad-SLI with a pair of GeForce 7950 GX2 cards, NVIDIA developed a rendering mode designed to leverage the power of the four GPUs powering the cards.
Typically, multi-GPU scaling is achieved by using either AFR, for Alternate Frame Rendering, or SFR, for split frame rendering. For older GeForce 7950 GX2 Quad-SLI configurations NVIDIA combined these two modes of operation and created AFR of SFR. What this mode did was take alternate frames and split the frames for rendering across the four GPUs. SFR, however, doesn't scale as well as true alternate frame rendering and SFR isn't universally compatible either. SFR also usually has more communications overhead and applications with a heavy vertex workload don't benefit as much.
With a pair of GeForce 9800 GX2 cards, however, NVIDIA is now able to use AFR in many games for better scaling across the GPUs. AFR is the preferred mode because all of the 3D rendering workload is parallelized (pixel fill, raster operation, vertex transform, etc.) and each GPU can work on its own frame. AFR is not universally compatible with every application, but when it does work, performance is usually increased significantly.