A flat breaking pitch is remedied with “get on top” most often. How does this relate to hand stays behind the pitch?
When the hand slips off to the side on a fastball you get reduced velocity. You want the hand off to the side on a curve, but when the hand is below the ball and the fingers are pointing up you tend to get only sidespin on the ball and it generally doesn’t break that much. When you get the fingers rolling over the top of the ball and pointing in toward your face then you tend to get more overspin on the ball and a sharper break.
A curve’s break also depends on the initial trajectory and the speed of the pitch. A curve released near level with mediocre spin can break more and more sharply than a curve released more upward with good spin.