Mirror cutting is a handy features for bending objects along whichever axis you want. A typical use might be to creat something like a picture frame. To use, select an object then select Polyons>Mirror cut>options box. You'll need the option box to select the settings for the tool. For bending purposes you'll want the combine meshes option selected. The manipulator handle for this includes movement and rotation option. You're actually moving or rotating a cutting plane which defines where the mirroring will occur. In the example a below a simple elongated box is mirror cut to create a 90 degree bend. To ensure a perfect bend, 45 degrees was specified in the appropriate channel box field.