this is a really simple exercise to make it harder you can either hold your hands above your head arms straight out or can hold weights in your hands as you do this. to make this easier hold your hands across your chest.
With the hands by your head as shown in this picture try not to pull your head up your hands should just be placed by your ears.
As with any core exercise the number of rep's you will need to do per set to get results is higher aim for sets of at least 20 and up to about 40 at the most if you can do these easily you need to be doing a harder variant of this exercise.
This is a great simple exercise that works your transverse abdominals or the 6 pack muscles as well as the other core muscles as stabilizers.