I believe the point at which the author leaps from a clear and beautiful worded explanation into deep complexity without accompanying explanation is the point a

I believe the point at which the author leaps from a clear and beautiful worded explanation into deep complexity without accompanying explanation is the point at which they no longer fully understand it themselves. Having attended many a conference and spoken to many an author, the deep bit of maths in their publication without any understanding is most often the bit you later find they copy and pasted.