How to strike a knuckle ball
The "knuckleball" is when the ball does not spin as it flies through the air. Spin is what gives a ball stability as it at moves, and without it your shot will snake through the air, changing directions suddenly and making it nearly impossible for a keeper to predict. A good knuckleball is all in the follow through, and with enough practice you'll be hitting them like Gareth Bale or Cristiano Ronaldo in no time.
Use a well inflated, full-size ball to practice. You want it to be nice and hard, and it absolutely must be regulation size. Hitting a knuckle requires you hit the ball firmly near the center and resist following through -- a flat, small ball will be easy to hit off to the sides and it will stay on your foot for longer, causing spin.
Perfecting this shot takes a lot of practice -- so having 5-10 balls to hit in a row will make your life much easier.
It doesn't matter what brand of ball you use to practice, as long as it is full-sized and well pumped
Set up for your normal free kick run-up. If you normally take four steps back and two to the left, you should use the same set-up here. The only difference would be to stand upright, keeping yourself balanced on the balls of both feet. As you hit the ball, you want to keep your chest up straight as much as possible, so it is best to start like this too.
Note how knuckleball experts Cristiano Ronaldo and Gareth Bale stand before a free-kick. They stand tall, with their chests almost puffed out.
Approach the ball like you would for a normal strike with your laces. Keep your torso up, but otherwise approach the ball like you were taking a normal free-kick. You'll be hitting the ball with your laces for maximum power and to avoid spin
Place your plant foot even with the ball, roughly six inches to the side. After your run-up, you want to place your non-kicking foot as close to the ball as you can get it. Your instep (where the inside edge of your foot curves slightly) should be in the middle of the ball. Your toe needs to point in the direction you want the ball to go
Strike the ball with your laces, right below the very center of the ball. You want to hit the ball as close to dead center as possible, going just slightly lower than center to hit the ball higher in the air (to, for example, get the ball over a wall).
Stop your follow through as soon as you hit the ball. This is the key to a good knuckle, and the hardest part to master. You do not want to follow through on the ball anymore than your momentum makes you. As soon as you make contact, stop driving your leg. You'll feel your shin continue through the ball, but everything above the knee should be mostly quiet. Some players find this easiest if they hop after the strike. As you make contact, pop off your plant foot. Land on your shooting foot first, placing it right where the ball used to be.