“From opposite its nape” — opposite that which faces the nape, i.e., the back of the neck. The phrase locates the cut on the rear side of the neck, not on the throat in front.
Type of Learning: Diyuk of the preposition “mul” (opposite): it specifies the side of the neck that confronts the nape.
What Is Learned: Melikah is performed at the rear of the neck. It is not the ordinary shechitah cut across the front of the throat; the bird is killed from behind.