Thanks for answering my questions. I'm sure you have better things to do!
My lecturer was saying that if you just had a flat underbody, low to the ground and no diffuser, the boundry layer could get thick enough to start to choke the flow. Thus reducing downforce compared to having a diffuser.
Am I right in this flollowing image, showing the pressure difference between a flat underbody and having a diffuser?
Can I ask, what do you do that means you understand this so well. Are you an engineer or scientist?