This modifier is similar to the optimize plugin. The difference is that it works on splines. It tries to approximate the given spline with a spline that looks the same but has fewer knots. You specifiy the maximum error and an angle for corner detection. It works both on bezier points and on corner points. So you can use it to 'reverse engineer' tesselated splines that have only corner points back to splines with bezierpoints. Its main purpose is to clean up data from imported dxf and dwg splines.