relaxation techniques control anxiety levels (athletes want to have optimum arousal levels). they (relaxation techniques) only work if arousal/anxiety levels are too high, they will have a detrimental/negative affect on performance levels if the athlete is under aroused... for example if a gymnast is over aroused and has a high level of anxiety before a vault routine then he/she might use visualisation or mental rehersal to mentally picture a successful perfromance/outcome or part of the routine (landing, run up etc) to allow the athlete to have an optimal arousal level... thats just off the top of my head, hope it helps, if i doesnt or you need more info just ask.