The Edge Weight tool allows you to add creasing to subdivision surfaces without adding additional geometry by creating a "weight" on various edges. To use the tool simply select an edge or edges and click on the Edge Weight Tool button in the Vertex Map menu and then click and drag in the 3D viewport. Setting the value to 100% will create a completely hard edge whereas a value of 0% allows the default interpolation of the limit surface through the control vertices. It is also possible in MODO to set edge weights to negative values which will push the limit surface away from the control vertices.
This tool is actually using the generic "Weight Tool" but first selects the Subdivision Vertex Map so that the appropriate vertex map is adjusted when using the tool.
