Hiraken is a somewhat old fashion way to strike. You don't see these kinds of techniques being practised too much nowadays. Maybe you never even heard of it. If the latter is the case, I hope you will be just as enthusiastic as myself when I saw this technique for the first time in Ishikawa Sensei's book. Anwyay.. will share the meaning of Hiraken first.
Hira (平) means flat and Ken (拳) means fist. Sometimes it can still be difficult to get a good translation after checking the Kanji, but in this case it is pretty straight forward. Hiraken means flat fist. As your hand is very versatile, this is one of your options.
Hiraken is a Shuto with the wrist kept straight, the fingers forcefully bend and the thumb on the side of the hand. The middle (proximal) joints of the fingers are used to strike. Hiraken is excellent to strike the sides of the neck, the throat and the side of the body below the floating rib.
There are other points to strike, that you should study carefully yourself.