Arguably, the proportional form of Bayes's Rule is the fastest way of describing a Bayesian argument, without using any special notation like odds ratios, which at least sounds like it maybe ought to be true\. If you were having a fictional character suddenly give a Bayesian argument in the middle of a story being read by many people who'd never heard of Bayes, you might have them say:
Confused about the role this clause is playing. What does "which" refer to --
the proportional form of Bayes's Rule?
the whole rest of the sentence?