Hi Martin,
Thanks for the explanation, and in fact the way I made my kind of helmet was to test this.

If I had to sculpt this shape I won't have any problem as I'll be able to resolve it while I'm sculpting.
My issue here is how to translate this same process to MOI.
If I trace the cut in 3D space and build my shape from many curves in 3D I'll loose the advantage of using nurbs and I think I will have a blobby result and have a hard time to make it smooth, the very same issue you can have in polymodeling if you have too much edges.

What I have in mind at the moment is how to find a way to buid my shape while still having it to support the cuts.
I have to test this out ;) I wish Lyed helmet tutorial was still available, too bad to see these kind of ressources disappearing because of bad hosting.