naturalization in boot, how does it work?

Hi there,
I know that one can join navy as permanent resident (with green card). I was told that your rating choices are quite limited this way. I was also told that there is program to allow naturalization in boot camp, so you start as a green card holder and you graduate as US citizen.

Is there anyone who can fill in details? How does it technically work, when you are not allowed to have any free time, any visitors, any documents and paperwork.?? I am really puzzled.