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; 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 has also to support this feature.
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). Your video card is supported for this.
Both HybridSLI modes are currently available only under Windows Vista.