The key component affecting accuracy of a throw is the path taken by the arm as it extends to the target. Since projectiles go straight with only gravity pulling them downward, we should release the ball with a vertical arm path so that we can actually use gravity as a deliberate factor in our throw, and avoid missing targets to the left or right. The arm essentially uses centrifugal force to accelerate the ball, and if that path is layed down horizontally, like a merry-go-round, we basically create 89 degrees of error on BOTH sides of our target, hence wanting to use the “over the top” release people talk about.
This vertical plane is actually known as the Saggital Plane, the line between the QB’s belly button and his target, and therefore the flight path the ball should take to ensure accurate lateral targeting.
Simply put, a “horizontal” delivery, of any degree, creates a timing problem: let go just a little early or late, and the QB misses his target laterally. A “vertical” delivery has the same timing variables, but letting go early or late only affects the ball’s vertical trajectory, making it go high or low, a much larger window of opportunity for a receiver to make a completion, since he can receive a ball from the ground to his highest point of jumping and extending: a high or low ball is not ideal or pretty, but still can move the chains or put points on the board, while a ball that misses just a few inches behind or in front of a receiver, regardless of how pretty, is just a pretty incompletion.
If you don’t think a few degrees left or right makes a difference, think again about the nature of angles. A throw at ten yards, that is left or right of the target by 1 degree will be off by 1.7 yards, or about 5 feet (of course a receiver can still make this catch). However, take the distance of the throw to 20 yards, and he misses by 10 feet, 40 yards away, the QB misses his target by 20 feet, or almost 7 yards. Considering a throw from the middle of the field to the sideline is over 25 yards and a QB will usually throw about 7 yards behind the line of scrimmage, a simple 10-yard curl becomes a 30 yard throw — just 4 degrees off, and you miss by 7 feet.
The following video from ESPN’s Year of the Quarterback and Sport Science‘s analysis of Andy Dalton, illustrates how one degree of error left or right on a twenty-yard throw results in a miss. (The video will continue to play after the initial piece on angular accuracy and address voice pitch and decibel. Feel free to stop video after the 1:44 mark.)
The following video from ESPN’s Year of the Quarterback and Sport Science‘s analysis of Aaron Rodgers, demonstrates the small margin of error allowed in order to make a completion.