A Magnus rise ball works by creating higher air pressure on the bottom of the ball in order to push it up. The most efficient way is to have the seams orientated so that for each rotation of the ball four seams pass directly under the bottom of the ball moving towards the catcher (like a 2 seam fastball grip). This is called 6/12 spin (as viewed from the catcher's perspective) and can be seen using 90, 0, 12:00 P.M., 0, 3:00 in the Spin Visualization Tool
The index finger, and also the ring finger, should be touching and pushing against the seams on each side of the horseshoe. A lot of pitchers curl their index finger a bit. Rotate the ball like turning a door know. Some pitchers pull their thumb off of the ball and basically grip it with just the index finger and the 4th finger which is on a seam. Experiment with the various grips to find what works for you. The index finger should cut off the bottom of the ball as opposed to going up in front of it. Practice the spin to have the stripe on the ball be parallel to the line of flight for pure 6/12 spin.
When doing drills many pitchers can get the good 6/12 spin. But it is very difficult to get the spin at 6/12 when throwing full pitches and many pitchers, when throwing the rise in games, actually throw a gyro ball (90, 0, 6:00 P.M., 90, 9:00). The gyro has no break other than gravity but when compared to a high fastball that has some drop it can look like it rises.
I used to teach the rise ball like that but I have switched to the SSW rise like most of the current top college pitchers throw.
