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Okay, let me give this a shot.
With all pitches the primary differences have to do with the velocity of the ball, and its rotation, and it's flight path. The positioning of the stitches with relation to the motion of the ball is also important - this, after all, is the difference between the two-seam and four-seam fastball.
With any pitch thrown, as the ball flies through the air, it leaves a "wake" behind it as the ball flies through the air. Theoretically, were the ball not spinning, the wake would be roughly symmetrical. Since it does spin, the wake is deflected to one side (the side where the spin is counter to the motion of the air rushing past).
Now, with the four-seamer (your basic fastaball) you have 4 seams rotating and disturbing the air about it during it's flight causing it to rise slightly. As it happens, the force generated by a vertical backspin is counter to gravity, making this the 'rising' fastball.
But with the two-seamer (the sinker), since you do not change the arm angle or release point as with other pitches, the only difference is the number of seams deflecting the surrounding air during it's flight. With only two seams causing air resistence the flight path of the ball differs slightly. With less air resistence the two-seamer is more directly acted upon by the effects of gravity along it's short flight path causing it to sink slightly.
I hope that makes sense.
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