First, let’s look on how a dart flies. It travels along a parabolic curve, the same curve as for instance a thrown stone or a rifle bullet uses.
A dart travels in a parabolic curve
The curve can be higher or lower, this depends only on how powerful the dart is thrown. A decent throwing technique must guide the dart exactly along this parabolic curve when accelerating the dart, and must guarantee that the dart can continue this curve when it has left the hand.
How must the dart be moved to keep it in the right position? To work this out we have to look at the mechanics of the throwing arm. It can be exactly described as a ‘machine’ of 3 levers attached to each other by 2 joints or ‘hinges’, and with 1 joint attaching it to a fixed point:
The highly useful multi-purpose lever-system of the human arm
Looking at the above image, the 2 attaching joints are elbow and wrist, the fixing joint is the shoulder, while the 3 levers are the upper arm, the forearm and the hand.
Those of you who had the misfortune of being tortured by mechanical science in school will remember that this is a very neat arrangement: It can theoretically draw every possible curve within its range when the levers are moved properly, and although the human arm is slightly less movable the parabolic curve is still an easy exercise.
The above image already shows the ‘aiming’ position when throwing darts. In the following animation you can watch how the ‘levers’ and ‘hinges’ work in a decent darts throw, keeping the dart exactly along the curve.