If you know how to, and if you practice, you can use playing cards a weapon. I am not going to teach you how to do that. In this Instructable I will show you how to throw them. Also, keep aware that you will destroy and lose many, many cards while doing this.
Just to mention, my camera is not great and I find it difficult to take photos with one hand, that is the wrong hand, and the camera is designed for the other hand.
*Ok guys, update! As it turns out, this way is wrong. It works, but is apparently my own adaption of a different way. Without overcomplicating it, your arm should be vertical, not horizontal.*
Step 1: The Basics
To hold it, grasp it between your thumb and middle finger, with your forefinger on the corner. Your two grasping fingers should be about 5 cm (2 inches) from the corner. This may sound confusing, but the pictures make more sense.
To start, you should sit on a chair, preferably a hard one. You should sit on the edge with your deck of cards on your lap. Start by holding your arm parallel to the ground. Your arm should be at a right-angle and be parallel to your torso. Bend back your wrist another 90 degrees so that it is pointing towards you. Yet again, this may all sound confusing, but the pictures explain it.
Step 2: The First Throw
Keeping your arm in the same position, flick your wrist forward and release in a smooth movement. The card must roll of the forefinger, as it's the rotation of the card that allows it to cleanly pass through the air. Once you can throw it distances without it immediately fluttering to the ground, you can move to the next step.
Step 3: The Arm
Step 4: The Stance
Eventually you'll want to stand. You should be able to throw a fair distance before doing this. The stance is pretty basic, with your feet apart and perpendicular. Your leading foot should be pointing to where you are throwing.
Your arm is still perpendicular to your torso, but now you throw a full 180 degrees. Before getting to do anything tricky, you should be able to throw 6 metres (20 feet) consistently.
Step 5: The Tricky Stuff
You have "The Flick" which is the basic version here. It will irritate people, but will not hurt them.
You have "The Butterfly Swirl" which is held in a much lighter grip, except the forefinger cocks against the corner harder and makes it spin faster. This will sting people, and it is often compared to the sting of a bee.
You have "The Sea-Urchin Spin" which is like the basic version, but the arm is held at a 45 degree angle towards the ground. As you are releasing it, you push down- which actually makes the card move upwards, and with more force. It is compared to the sting of a sea-urchin's spines and can draw blood if bare skin is struck.
You have "The Dolphin Dart" which is very powerful. The card is held at neck level this time and is thrown with a powerful motion of the arm. If you're good enough, the card will make a cracking sound as it is thrown.
You also have "The Lethal Four Finger Fist" which I cannot do and do not know the technique for. All I know is that you throw four cards at once.
Step 6: Final Note
Step 7: The REAL Version
As your arm comes down, you have to flick your wrist and create the spin. As you get better, you will notice that your wrist flicks backwards and away from the card, this adds a lot more spin.