A sinker and a two-seam fastball have some similarities, but it depends on how the pitcher throws it. Both sinkers and two-seamers fastball pitchers leverage their middle and index fingers on the seams when it comes to the grip. The pitcher’s arm movement and how their hand works through the ball can bring a difference. When pitchers throw two-seamers with an arm-side lateral movement, it is merely a two-seam fastball, but if they throw a two-seam that has a significant sinking action, that is a sinker. A sinker has more diving action than a two-seam fastball.