How it works: it lines the two versions up using the longest common subsequence, which is the same approach behind the diff built into version control — so a paragraph inserted in the middle shifts everything sensibly instead of marking the whole rest of the document as changed. Where a line was reworded rather than replaced, the changed words inside it get picked out too, which is the part that matters when somebody quietly swaps thirty days for sixty in a contract. Every bit of it runs in this page. That is worth saying plainly, because the usual way people do this is pasting a confidential draft into a stranger's website.