The functionality was improved and called QuickDiff.
Original functionality:
When a lengthy or important page shows up in RecentChanges you may wonder what change put it there. You can get a line by line list of differences. Here's how...
Wiki runs a unix diff between the two versions and puts the output, if any, in the Differences section. A page authored from only one site won't have a backup copy so you won't find an EditCopy button. In this case there is nothing to compare.
Here's an idea: could you put an around the Differences section, such that a script could cause my web browser to go straight to there? -- GrahamHughes