The key to the argument lies in the point we established earlier about 'linkage disequilibrium', the 'togetherness' of genes for tails of a given length - any length - and corresponding genes for preferring tails of that self-same length. We can think about the 'togetherness factor' as a measurable number. If the togetherness factor is very high, this means that knowledge about an individual's genes for tail length enables us to predict, with great accuracy, his/her genes for preference, and vice versa. Conversely, if the togetherness factor is low, this means that knowledge about an individual's genes in one of the two departments - preference or tail length - gives us only a slight hint about his/her genes in the other department.
The kind of thing that affects the magnitude of the togetherness factor is the strength of the females' preference - how tolerant they are of what they see as imperfect males; how much of the variation in male tail length is governed by genes as opposed to environmental factors; and so on. If, as a result of all these effects, the togetherness factor - the tightness of binding of genes for tail length and genes for tail-length preference - is very strong, we can deduce the following consequence. Every time a male is chosen because of his long tail, not only are genes for long tails being chosen. At the same time, because of the 'togetherness' coupling, genes for preferring long tails are also being chosen. What this means is that genes that make females choose male tails of a particular length are, in effect, choosing copies of themselves. This is the essential ingredient of a self-reinforcing process: it has its own self-sustaining momentum. Evolution having started in a particular direction, this can, in itself, tend to make it persist in the same direction.
