That's called a cowboy. You tuck super hard with bent knees, spread your knees a little and tuck your head. It was started in floor gymnastics to protect your head and neck in flips, as well as add style and speed.
I was on my schools dive team all four years. As for practice, I did 2 things that helped me SO MUCH:
1. Go to a tramp gym, if you can find one. On the running tramp (the long, skinny one), do various rolls. Cowboy drops, peanut rolls. Ask a coach for some examples. It will help you learn the positions you need to be in.
If you can't find a gym, practice cowboy drops on a friends big trampoline. Bend in half and drop onto your back in the position I described above. You'll need some momentum so you can flip back onto your feet from your back.
2. This was the most helpful thing I did in diving: After you figure out how to do the dive you want to do, go to the edge of the pool and drape your elbows in the gutter or over the edge. Practice the exact movements you need to do in your dive over and over again. Practice tucks, straight dive positions, and arms. MEMORIZE the EXACT MUSCLES you engage.
If you are doing a pike, for example, bend in half. Remember the way your hamstrings pulled and how flexed your quads were. Remember how tightly you had to hold your abs in order to hold the position. Remember what your upper back did and how you used your arms.
Muscle memory will be your best friend and core strength are your best friends when it comes to diving.
Good luck! PM me if you have any questions.