It's also about string oscillation. The point that your hand is putting pressure on the string is farther from the point that the arrow's mass is sitting on the string. When you release, the string begins to move forward. It takes a fraction of a second longer to get the string behind the arrow moving than the est of teh string. This creates wave, or oscillation in the string. Happens to a split finger shooter too, but the oscillation is less pronounced because the two points are closer together.