Having been an athletic coach for nearly forty years I can attest to the effectiveness of visualization on performance. I used it as a participant in the 70's and have done so with many athletes. My understanding when I first read about the technique was to visualize the successful completion of a task (in my case high jumping) prior to making the attempt. It was further posited that those who had a thorough understanding of the task should be able to visualize it. Should one be unable to visualize the task it was thought that the person had gaps in their understanding of the task that were responsible in great part for a lack of mastery. It worked for me in the 70's despite not having the plethora of video feedback options now available. Nowadays with my athletes if I find a particular technique weakness I can show it to them by video and add visualization correctives.