HybridSLI is an umbrella name for two technologies: GeForce Boost and HybridPower.
GeForce Boost is a technology targeted to entry-level PCs with on-board video to increase 3D performance. On motherboards supporting this technology – basically motherboards with on-board video based on NVIDIA chipsets (not all NVIDIA-based motherboards support this feature; see the list of motherboards in the next page) – when you install a “real” video card you can configure the on-board video and the video card to work together in parallel under SLI, increasing the 3D performance.
The problem is that besides the motherboard the video card also has to support this feature and so far only GeForce 8400 GS and GeForce 8500 GT support it.
HybridPower, on the other hand, is targeted to very high-end systems. This technology turns off your video cards when you are not playing games, thus saving energy. Under this situation video is provided by the chipset (i.e., on-board video). This looks like a terrific idea, but the problem is that so far only two video cards are compatible with HybridPower: GeForce 9800 GTX and GeForce 9800 GX2. So unless you have these video cards this technology is useless.
Both HybridSLI modes are currently available only under Windows Vista.