Any discussion of a flathand must start with flexibility! To achieve the best flathand possible and a great rip entry, a diver must work on the flexibility of the wrists.
Without that flexibility, a diver's hands will look more like a ball and it will appear as if they are punching the water with their fist. Have you tried to punch the water before? Give it a try, you will find out that it is not a very effective method of getting in the water without a splash!
So practice that flexibility every time you stretch before and after practice. You are stretching before and after practice aren't you?