The knuckle curve is implemented often when a pitcher struggles to throw a traditional curve ball.
It's done so by holding a curve ball grip and then sticking or spiking a knuckle into the baseball. And this will slow down the curve ball creating a slower, more tumbling effect on the pitch as the ball will tumble in a 12 to 6 configuration as the hands on a clock. Or also known as a nose to toes dropping manner.
When throwing a knuckle curve, the pitcher should also feature a high fast ball so that the batter's eye level is mixed up. In doing so, you may confuse the batter, especially in a two strike count. Many young players will implement a knuckle curve ball because throwing a traditional curve ball is a struggle for them.
This is how you throw a knuckle curve.