In fly casting, acceleration refers to steadily increasing the speed of the casting stroke in order to build a corresponding velocity in the line. In turn, the accelerating line “loads” the rod by putting a progressively deeper bend in it as the stroke advances to the stop at the end of the cast. Furthermore, acceleration must be made smoothly so that the line is free of any shock-waves. When properly executed, the rate of acceleration will reach its zenith at the very end of the casting stroke. Acceleration does not mean that the rod should be moved as fast as possible from beginning to end, nor does it infer that the rod travel at the same speed throughout the stroke as both of these inefficient techniques lead to poor loop formation.
Near the end of a casting stroke and immediately preceding the stop, the rod must undergo a brief transition from pointing in one direction to pointing in the other in order to complete the cast. (As examples: On a forward cast, the transition would be from the tip facing backward to forward; or, during a backcast, from forward to backward.) During this transition, or “turnover”, the speed of the rod will increase exponentially into a stop. This transition has been referred to by several names. Some of them, such as the terms “speed-up-and-stop” and “power stroke” imply a separate stage of the cast; however, the turnover must be a smooth and seamless part of the entire acceleration phase. If you use the grip I recommended in an earlier article, the space or “gap” between the rod butt and the wrist will completely close during the turnover on the forward cast. My old friend Dave Johnson refers to this as “closing the gap” and I borrow his highly descriptive phrase and use it interchangeably with “forward turnover” whenever I have the opportunity. Efficiently closing the gap makes the turnover a sharp one and helps create a tight loop in the process. For clearer insight into the acceleration phase including turnover, take a look at the attached video.