Just like a regular block, but has a way to be pulled into other blocks.
Example: "Aw, I have a bomber fuselage with no bomb bay.. its gonna take me ages to make another one.. Oh wait! I have the new Negative Block option! I can just simply drag this block out of the menu, place it on the plane and then double click (Like the wing editor except it drags the part) and drag it where I want the bomb bay to go! Look at that! I have a hole in my fuselage now! Thanks Negative Block option!"