SketchUp will Autofold faces automatically when a move or stretch operation will create non-planar faces. For example, clicking on the corner of a box with the Move tool and move down in the blue direction causes SketchUp to create a fold line along the box's top face.
Forcing Autofold Behavior
There are times when SketchUp constrains an operation in favor of keeping all faces planar and not creating additional fold lines. For example, clicking on the edge of a box with the Move tool only allows you to move the edge in a horizontal direction (red and green), but not vertically (blue).
You can override this behavior by pressing and releasing the Alt (Microsoft Windows) or Command (Mac OS X) key before performing the move operation. This key sequence enables Autofold allowing geometry to move freely in any direction.