As I indicated earlier, the prerequisite for that slip pitch is a good slider.
Let me refresh your memory on how to throw it: You grip the ball with the index and middle fingers off-center and very close together, with the middle finger just touching one seam. As I was told, you throw it like a curve ball but with an easier wrist action—you just roll your wrist, don’t snap it. You can practice it by throwing the pitch with your normal curveball action and then ease up on it, and you should get a good break on the pitch. I remember how I threw my curveball with a sharp karate-chop wrist action and had to get used to easing up on it, but eventually I got it.
For the slip pitch, you can use either a two-finger or a three-finger knuckleball grip—which one you use will affect the break. Basically you throw the hard slider with it, which makes this pitch a fast-breaking one. And once you have it, you’ll find that it really screws up the batters’ timing—and their thinking. I remember how I used to have fun with it, especially when I used the crossfire (I was a natural sidearmer). And incidentally, pay no attention to the negative comments—the person who posted them was talking through his back end. 8)