a) There is no difference in movement between a 2 seam and a 4 seam pitch. There is a difference in perception of the two pitches. But, the actual flight paths don't vary.
b) In baseball, some pitchers throw a 2 seam and 4 seam fastball with different arm slots. If so, then there is a difference in movement *in baseball*. But, in softball, a pitcher can't vary the arm slot.
Wasn't this covered earlier in the thread?
Slug, I was with you initially but JJ showed me the error of my ways. It's a question of symmetry during the flight path. The 4-seam has symmetry at any orientation where the 2-seam does not.
Having had DD practice with it (and me on my bucket), I can say anecdotally that there is actual movement that doesn't happen with a 4-seam pitch. We're going to work hard on it during the off-season. When she threw it well it tailed away from a righty, and coming from a lefty pitcher that should be a pretty good out pitch!