The Undercut Analysis 
tool finds trapped areas in a model that cannot be ejected from the mold. These areas require a side core. When the main core and cavity are separated, the side core slides in a direction perpendicular to the motion of the main core and cavity, enabling the part to be ejected.
This topic applies only to CPU-based undercut analysis. The software uses CPU-based processing only if your graphics card is not able to perform the necessary GPU-based processing required for the newer GPU-based undercut analysis. See Graphics Card Considerations in Mold Analysis Tools.
Undercut Analysis works only on solid bodies. It does not work on surface bodies.