What does 'As diffuse' do in the Glossy material?

This option only works in photon mapping. When is enabled, the diffuse photon map is used to calculate the glossy reflection instead of ray tracing around. The glossy reflection will be calculated faster, but with less precision, due to the approximate nature of the photonmap. Therefore, this option is advisable for glossy reflections with a low exponent (high blur). Also this option enabled will tend to produce reflection flickering in animations.