Two-seam fastballs are thrown with slightly less velocity than a four-seamer (generally 2-4 mph less), but have more movement and sink.
Pitchers throwing two-seamers must find a balance between higher speeds and movement. It should not have the egg-like pressure with fingertips and should not have any palm exposure, similar to a change up.
The two-seam grip is two based fingers with the thumb underneath the ball in between the index and middle fingers, resting at the top of the ball. Both fingers should ride with the seams and should be applying pressure over each seam. The most amount of pressure should be at the fingertips and slightly past the knuckles. Increasing or decreasing the pressure in one finger over the other helps slightly differentiate the movement of the ball.
Typically, there will be more sink if a pitcher applies more pressure over the middle finger rather than the index finger. Some guys will even pronate the hand slightly upon release to exacerbate the sinking action.