This mode takes 4 pics and then processes them. The resulting image is better than what I get if I use the low noise S/N EXR mode. Does anyone know what it does?
It just averages the images together. Since noise is largely random, it is very effectively reduced by this method and you will see improvements even at base ISO. You can get the same function from shots layered in PS or a number of standalone programs like Image Stacker and various astronomy programs. You get diminishing returns past about 4 shots. Typically 8 shots is only a little better, and 16 shots only a tiny bit better. Great technique for night shots.