How SLI Works
There are five SLI rendering modes available: Alternate Frame Rendering (AFR), Split Frame Rendering (SFR), Boost Performance Hybrid SLI, SLIAA and Compatibility mode. In practice, however, you can forget about the latter four. Modern games almost exclusively use AFR.
AFR, in Nvidia's own definition, is:
[In AFR mode] "the driver divides workload by alternating GPUs every frame. For example, on a system with two SLI enabled GPUs, frame 1 would be rendered by GPU 1, frame 2 would be rendered by GPU 2, frame 3 would be rendered by GPU 1, and so on. This is typically the preferred SLI rendering mode as it divides workload evenly between GPUs and requires little inter-GPU communication."
In order to benefit from performance scaling, individual applications need a so-called SLI profile that tells the display driver what specific form of synchronization is required, and which others can be skipped. Nvidia creates these profiles and releases them as part of its periodic driver upgrades. Newer games may not have SLI profiles available when they launch, or the SLI profile initially released may be buggy (creating visual artifacts) or not yet optimized (limiting scaling).
For more information, please refer to this white paper published on Nvidia's developer network. We encourage all of you who wish to learn more about the technology to read it: SLI_Best_Practices_2011_Feb.