The Dunker is my improved version of a tuck cast. I use it when my nymph has to go really deep in a short time. This might be the case if you want to fish a pocket water behind a rock close to a fast current.In such a situation it is often necessary to make the nymph sink very deep before the current pulls it away. I named the cast after the (slam) dunks in basketball which force the ball to pass the ring directly from above on high speed. There are many versions of the tuck cast around but they do not follow the biomechanical principles in a perfect way. That means the power vectors are not pointing exactly in the direction they should. The perfect tuck cast makes a nymph hit the water very hard and the leader is following the nymph vertically. That means a pile of leader is landing on the same spot and supports the sinking of the nymph. As a result the nymph is reaching the fishing position much faster.
To cast a correct Dunker it is necessary that the line is cast in a vertival plane. This makes it possible that the nymph dives completely vertical, too. If the nymph hits the water from the side, power is lost and the sinking speed is reduced.
how you cast a Dunker:
Flase cast your nymph in a vertical plane(use a little bit a larger loop than with a dry fly) and during the last casting stroke you speed up the nymph by applying more speed on both, the rod and line hand. You stop the rod hard in the 11 o'clock position and immediately(parts of a second) after the stop you lift the rod-tip up and slightly forward with very high speed. This makes the line (and nymph) being pulled around an imaginary wheel and speeds it up after the stop and thereby alters the angle in which the nymph hits the water in a positive way. The nymph lands in a nearly 90° angle to the water surface. On distances exceeding 8-10 m I use my Vertical Bow Cast.