From
Gameday: PFX vs. BRK and Pitch Types:
"Pitch-f/x" is the distance between the location of the actual pitch thrown over the plate, and the calculated location of a ball thrown by the pitcher in the same way, with no spin. Or, in more common terms, this is the amount of "movement" the pitcher applies to the pitch.
Notice that the trajectory with "no spin" is slightly diferent than a trajectory in a vacuum, because in vacuum the ball wouldn't slow down on the way to the plate. Pitch-f/x calculates the trajectory including the drag which slows down the ball. The drag is in the direction opposite the instantaneous direction the ball is traveling, so it assumes that there are no aerodynamic forces are pushing the ball perpendicularly to the direction of the ball.
So, when a pitcher throws a 100mph pitch with a pitch-f/x of +12, it means that the pitch only fell about 3.5 feet on its way to the plate, rather than the 4.5 feet it would have fallen if it had no spin at all. For a normal pitch to qualify as "rising" (more inches above the plate at the back of the strike zone than at the front), it would need a vertical pitch-f/x value in the neighborhood of 60 inches. Maybe Chad Bradford (a submariner) could get by with +25 inches or so, assuming he could figure out a way to get backspin on the ball while throwing underhanded.
Gameday also uses the term break:
"Break" is the greatest distance between the trajectory of the pitch at any point between the release point and the front of home plate, and the straight line path from the release point and the front of home plate.
An eephus pitch would have a lot of break, using Gameday's definition, because the ball has a huge arc on the way to the plate. Since such a pitch is typically thrown with little spin or velocity, it would likely have a pitch-f/x value near zero.