The sudden and sizable valuation markdowns of high-flying private companies like Snapchat, Dropbox and Zenefits by mutual fund investors has been one of the biggest stories in tech, sowing fears that the market is deflating. They’ve also caused consternation among founders, who have had to defend their companies’ earlier valuations to employees.

Yet the markdowns are one of the least understood phenomena in tech, as the funds are tight-lipped about how they come up with those valuations and why they change.

A close reading of publicly filed disclosures, however, sheds substantial light into what’s happening, and how funds are valuing major private companies like Uber, Didi Kuaidi, Palantir Technologies and Snapdeal.