Was just looking for that myself...
"Looking at the blade, you may think that the raised teeth are for cutting however, they don't cut the grass at all. The angle of the teeth is what produces those clean-cut mulching results. The teeth angle toward the center of the blade to redirect airflow and push the grass clippings back over the cutting edge of the mower blade repeatedly resulting in smaller and smaller clippings, giving you that high quality mulch. And because the teeth aren't used for cutting, you do not have to sharpen them and are able to sharpen the actual blade the same as you would with a standard blade."