Can anyone tell me how to throw a knuckle curve?
Yes, I can.
Place your fingers like a regular curveball on the bottom of the horseshoe. At this point your fingers will be index and middle on the left side of the horseshoe. Now, bend your index finger so that the nail of your index is touching the lace. It might feel uncomfortable at first, but now (for me) it feels just as comfortable as anything else. DO NOT flick your finger that is bent when you throw it, because trust me, it just doesn’t work (if it does your pitch will be about 10 mph)
Quick pro’s and cons.
Pros (for me): Moves more than an average curve, not to slow either
Cons: easier to read as a knucklecurve coming out of your hand because of the knuckle pointing up.