Guys...when I give my husband a bj why does he pull his balls down?

Okay so lately I have noticed that when I give my man a bj he has been pulling his balls down. He is uncircumcised. And I have noticed that when he pulls on his balls it does make his penis more firm(I guess it pulls back the extra skin or something) and bigger looking. We have been married for about three years and he has just started doing this recently like within the past month or two. I've noticed that it has made his balls sag a bit more than usual ... Lol sorry for the silly question but I'm just curious why he does it ...