The basic observation is that, since all/most aspects of a system are likely to be of high or low quality, you can use the quality of any of them as s proxy met

The basic observation is that, since all/most aspects of a system are likely to be of high or low quality, you can use the quality of any of them as s proxy metric for overall quality. Note that this fails if the proxy metric becomes popular or standardised in any way, since those providing the systems will realise they can appear to be better by improving only that proxy metric.