From what we know NVIDIA spent quite some time to explain to its investors and analysts why hybrid SLI is a good thing. The company plans to unveil such a product this year. It is a combination of IGP graphics and discrete graphics rendering together. The principle is rather simple. You take a graphics card, lets say a GeForce 8400 or a GeForce 8600 and plug it into the integrated graphics board and when you start playing 3D game the graphics card starts rendering together with the integrated graphics and ends up faster than the graphcis cards itself. This actually follows the multi-GPU logic, but now with different graphics chipsets working together to increase the frame rates in games. - Fudzilla

NVIDIA Has High Hopes For a Hybrid SLI Technology

If you don't play 3D graphics, the chipset can send your discrete graphics card into idle and save some power. Also AMD talked about such a concept and this sounds like a great idea for notebooks. According to NVIDIA, if you plug a GeForce 8400 series graphics card into the board, you should get a significant performance increase, but this mode won't give you a huge bump with a GeForce 8800 GTX, but it will be a few frames faster than without hybrid SLI. In the end it does sound like a great technology which certainly can bring sosme added value for the low and mid-range graphics card segment.