What is likely happening is that the pitcher first throws a fastball at one speed, and then, using an identical arm motion, throws another fastball at a higher speed. The higher-speed fastball arrives faster and sinks less due to its high speed. The added back-spin from the higher speed further decreases the amount of sink. When the pitch is thrown, the batter expects a fastball at the same speed, yet it arrives more quickly and at a higher level. The batter perceives it as a fastball which has risen and increased in speed. A switch from a two-seam to a fastball can enhance this effect.