The diffuser is not the engine undertray. Its a big piece of smooth black rubber that's molded to fit the underside of the front bumper. It fits like a lower lip and has both aesthetic and functional utility. I don't know why Nissan calls it a diffuser but that's what the parts gal researched for me. Once the diffuser is fastened onto the bottom of the bumper, the stock engine undertray can now be fastened to the diffuser. Without the diffuser, you won't be able to attach the undertray (either the Nismo or stock undertray).
My bumper is on already but I'll see what I can do about getting some pics posted. Might be a little while, as I don't own a digital camera. All my money goes to my car
I've attached a diagram that included the diffuser. Its labeled in red.