That one is easy...and not easy to answer...
Depends on your knowledge of Human anatomy.
The first gains are usualy inner penis "getting further out", or a dramatic reduction in fat pad volume by dieting, or both counts along with trimmed pubes.
The following size gains in lenght are brought on by cell division, new cells, structural elongation ( the actual stretching of inner structures ). Continuous tension of these structures translate into body adaptation to stress, it either breaks or it stretches. The cementing part of things is actualy the most important because if you do not overcome the innicial "rubbery" feeling of a longer dick by practicing maintenance routines before the gains are "cemented", you will revert back to starting point.
I suggest you read up on the stikies, those answer most of this type of question as well as many others you will have in the future.
Keep safe and enjoy this...
Peace
Mike