Hi Pilou - oh you mean the Sketch curve command.

Yeah that one is a bit different since it is all done with one kind of brush stroke.

That one always goes directly on the construction plane. If you want to sketch something at a particular angled 3D plane, the script mentioned here can be a quick way to set the construction plane to the current view direction:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2599.4


But probably Pierre was asking more about how the machine functioned for 3D graphics display and not really asking about how to use particular commands in MoI?

- Michael