Unattached voids in families (i.e. voids that don’t cut anything in the family) can now be used to cut objects in the project where the family is loaded. The objects that can be cut are: Walls, Floors, Roofs, Ceilings, and family instances of these categories: Structural Framing, Structural Column, Structural Foundation, and Generic Model. The main advantage is that you can parametrically control the cut size and features. In the below example I can control the Length, Width, Depth and chamfer size.
No in-place families have been created in the above model and one void family has been used to cut the Structural Wall, Strip Footing and the pad foundation. This is a much faster workflow when you need to cut multiple objects.
See my quick tutorial on YouTube for more information.
http://www.youtube.com/user/RevitStructureBlog?feature=mhum#p/a/u/0/vmSz5TVwX3U
Hope this is useful,
LawrenceH
Autodesk Revit Structure 
i try to use this , to cut at ramp, it not working.