The Boy Band Principle is basically another way of looking at pre-selection, using the popularity of boy bands as an example. Once the popularity level of a boy band reaches a certain tipping point, their popularity explodes. This is because, at that point, women begin to like the boy band regardless of whether or not they like their music, but simply because other women like them.
Similarly, in pickup, if a PUA is able to demonstrate that other women are attracted to him (through the use of pivots, stories, or subcommunication), other women will start to find him more attractive, simply because he is liked by other women. The same principle has been observed in many species of animals as well.
Evolutionarily speaking, it is believed that pre-selection is an unconscious shortcut that women take when evaluating the attractiveness of a man because it is generally an honest indicator that is difficult to fake. PUAs would do well to keep the Boy Band Principle in mind and take advantage of social proof and other social cues that people use to make snap judgments about unfamiliar people.