There are several versions of the Slider, but we will illustrate the cut-fastball version (aka a “Cutter”) because it’s the easiest for most people to learn and throw. The Cutter is gripped similarly to a Two-Seam Fastball (index or middle finger along the Blitzball seam), except the two fingers should be closer together and the ball should be held with an off-center grip (towards the outside half of the ball).
Throw the Blitzball just as you would a regular fastball, but with your two fingers positioned about a half-inch to the outside and the ball should naturally roll off of your index finger to the side when you release it (kind of like throwing a football spiral). This Cutter is thrown just like a fastball with an off-center grip and requires no wrist snapping of any kind. Just make sure that the ball rolls off of the thumb side of your index finger as you release it and the ball should curve sideways a good 3-4 feet (away from a righty batter if you are a righty pitcher and vice-versa).