This is very well practicing exercise in Karate and Jiu-Jitsu.
This Exercise helps developing "Iron Shirt", strong upper group of AB muscles to be able to hold punches and kicks in the area of stomach.
Mount Fintastica Android phone on Arm.
Put your hands behind your neck and lock your fingers.
Lift your shoulders, upper part of your body.
Your Target: 100 reps, fast.
Note that Karate Black Belt requires ability to make 1000 reps in one time.
My personal record is 250 :-)