Brian Wilson provides a great example of how pitchers keep hitters off balance by changing the level of their pitches. Wilson likes to work up with his fastball:
Note that Brian stays up on two levels. He throws very few fastballs at the bottom of the zone, but the pitch also drops less than expected from the batter's point of view. The pitches end high in the zone, but also higher than expected.
Wilson's slider does the opposite:
Very few sliders find the top of the strike zone, and they drop further than a batter expects. The combination of fastballs and sliders working on multiple levels keeps hitters off balance. They hit just .223 on the fastball, .229 on the slider.