This hda will identify edges that are flat, and dissolve them. The default behavior is to only find perfectly flat edges. The max edge angle slider allows a threshold of 'flatness', so edges which are mostly flat can also be dissolved, or the threshold can be set higher to obtain different effects, say artisticly driven decimation.
Max Edge Angle
Threshold of 'flatness'. 0 means edges have to be perfectly flat, higher numbers will find more edges and dissolve them.