Generally speaking in CINEMA 4D, filleting is done by subdividing edge polygons and adding HyperNURBS object. However it is not so easy to subdivide edges, with regurlar tools in CINEMA 4D.
With this tool, you can subdivide edges few times faster and also acculately.
While ON selected, you select 2 points, polygons between them are cut by knife.
At this time, knife is done along with the plane which includes both normals of the 2 points.
In case of no point selection, or number of selected points is not 2, this XPresso do nothing.
This XPresso tool can not handle "Undo". So please save scene before edit object with this tool.