Gives you the benefit of both the upcut spiral and the downcut spiral in one cutter. These cutters are great for faster feed rates when you have a laminate on both faces, or for when you are routing composite woods. The mortise length is used when not cutting through the material and a shorter cut length is required. With compression tools if the down spiral portion of the tool is not engaged with the material you will not be receiving the benefits of it being a compression spiral.