Gaining

At least in theory, the point where they go "Wow, I really am fat now, enough to really notice it, be slowed down and inconvenienced by it, fat enough that really I could stop gaining now and just enjoy being fat enough.....but I think I want more than being fat enough, I'm going to keep going."

I think it's when the belly folds over for the first time when standing...

I would say the first time both parties become aware of the feeder/feedee relationship and act on it. Knowing what is instore for the both of them. One watching, one eating, or even both feeding each other.
Oh... The excitement has to be intense!