I found this article from the NCAA about flip throw-ins.  I had to work hard to learn my flip throw.  My family always went to open gym at CATS in Boulder on Saturdays.  Lots of trampoline fun.  Then I met Coach Zak from CATS who trained gymnastics with Danica Evans.  I met her after a CU game and she told me I could learn it.  It took me 6 months before I  did one in a game.  I had to convince my Coach to let me try it.  It worked and I have been doing it every since.  I was 10 when I started doing them.  I have a video of me on a trampoline trying it for the first time.  It’s funny.