Article: The index-up grip is similar to that for a basic curveball, with the exception that your index finger remains relaxed and pointed up. This grip is good for beginners because your will use your index finger to help guide the ball towards its target. Use this pitch to practice your curveball technique, but avoid using it in games. Experienced batters are likely to see the raised index finger and adjust their stance in anticipation of a curveball. Bend your middle finger, leaving your index finger pointing upwards. Like in the standard curveball grip, let the bottom of the ball rest against the side of your ring finer. Place your thumb on the back of the ball so it lies along the bottom seam. Position your middle finger along the top seam opposite your thumb. At this point, your thumb and middle finger should form a "C" while your index finger is pointed up. Make sure your index finger is up and relaxed, and the ball is firmly gripped by your thumb (bottom) and middle finger (top).  While your grip should be firm, relax the muscles in your wrist and forearm. When throwing, your thumb rotates up while your middle finger rotates down. Use your index finger to control the trajectory of the ball - the ball will go in the direction in which your index finger is pointing.  Your index finger is not needed to throw the pitch, with the thumb and middle finger providing all the spin.  When throwing, your elbow should be at the same height or above your shoulder. Your hand and wrist should form a vertical line with your forearm.  Keep your forearm and wrist muscles loose until just prior to release. Strongly contract them and quickly snap your wrist forward and inwards, allowing the ball to spin out over the middle finger.

What is a summary?
Use a modified 4-seam grip. Hold up your middle and index fingers. Position your middle finger and thumb. Prepare to throw the ball. Throw by snapping your thumb and middle fingers together.