It occurs suddenly and relatively completely so that obvious changes in performance occur. But now they have become training items which means that athletes' progress is halted at a less than terminal stage of skill development and competing patterns of movement are established. When athletes develop faults, they need to be re-taught the element in question and the steps that follow that element. It is teaching the element in context of the preceding movements that is important. A device artificially trains competing movement patterns and introduces inefficiencies.
Many devices have no acceptable data to support their claims of benefit.
Most respectable research shows them to have no value or negative benefits. The body does not have the capacity to determine the intention of some training activity. Since most high level performers are discriminated from each other on the basis of skill efficiency, one of the most important factors for differentiating medalists from non-medalists, the level of performer skill should be maximized. Drill practices and the use of training devices work contrarily to that aim. That might be acceptable for individuals in the early stages of skill learning, but it is not acceptable for highly-trained individuals.