This is kind of the quiet time for softball...so, I'm throwing this out for discussion. A lot of this thinking is based on Riseball's many posts about "cutters". (For full disclosure, I didn't agree with what he was saying initially. In retropsect, he was right and I was wrong.)
Why is is important to understand these types of breaking pitches? Pitching instructors should be tell the athletes what they are "really" trying to do rather than the current gobbledgook served by most PIs.
The more I've looked at this and read Riseball's posts and looked at videos, it seems there are two groups of movement pitches. The first group is Magnus effect breaking pitches. The second group is seam orientation breaking pitches.
MAGNUS EFFECT MOVEMENT PITCHES
There seems to be a group of "major" movement pitches based on the Magnus effect. (The Magnus effect is that spinning balls thrown with a spin axis not collinear with the direction of movement curve.)
The major movement pitches are the riseball and drop. These are the common Magnus effect pitches. Backspin makes the ball fall less than a normal fastball. Top sping makes the ball drop more than a normal fastball. You can see similar movement in tennis, ping pong, golf, etc. There is plenty of video evidence for these pitches.
There is some video evidence of Magnus effect curve balls. (Curve ball is defined as a breaking pitch moving left to right or right to left. Note that this is *NOT* the same definition as a baseball curve.)
There is very little video evidence of Magnus effect screwballs. Perhaps someone is throwing one, but they are few and far between.
SEAM ORIENTATION MOVEMENT PITCHES
The other way to make a ball move is to have a non-symmetric seam orientation,
The most well known seam orientation pitch is the knuckle ball. The knuckle ball is thrown with very little spin. As ball rotates, the orientation of the seams changes. The result is that a well thrown knuckleball can actually change directions on its way to the plate.
In college softball and in the MLB, there are a lot of bullet spin pitches being thrown. Many of those The bullet spin pitches have a non-symetric seam orientation, making the ball move. If you watch slow motions of MLB pitchers, many of them are throwing pitches with bullet spin but with an asymetric seam orientation.
Attached is a example showing a bullet spin pitch in the "nose up" position. There is more drag (i.e., friction) at the top of the ball than the bottom of the ball. This causes the ball to have some lift.
Another example is the cutter. The image is attached. The view is from the batter. Note that there are more seams per square inch on the right side of the ball than the right. Since there are more seams per square inch on the right side of the ball, there is more friction on the right side of the ball. This causes the ball to move right.
This is similar to a shopping cart with a stuck wheel. The shopping cart tends to veer to the side with the stuck wheel, because there is more friction on that side of the cart.