Hi Ambimind,

> I'd love to know by what principle its possible to predict the final shape?

Well, for that kind of operation the shape is generated by the extrusions of each profile curve and then calculating the boolean intersection between those extrusions. Boolean intersection keeps only the area that is inside of each extruded volume.

So to get a better idea of how it works you can do the extrusion yourself with the Extrude command to see what kind of shapes are being used.

Some other examples of this method in this thread:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3174.4

- Michael