There's an OS API where you can register your app or window to get folder change notification messages or updates. I forget if it uses a Windows Message or what. Last time I used Locate32 I noticed when I set it to automatically detect changes, I got that Explorer opening every file in the system. I assume it used that API. If I turned the automatic change detection off, it didn't churn the drive but then I had to do manual updates, sort of like the Linux locate command.
Everything Search isn't perfect. Sometimes I notice on start it doesn't refresh the little database and I have to kill it, delete the .db file, then run it again. Now and then it happens. Other than that it's pretty easy on the drive. It keeps the .db in memory. I notice when I kill the app it writes the file to the home folder.
Of course on Vista pre-SP1 there was a lot of stuff that contributed to HD churning. SP1 fixed most of it. Even so, I turn Windows Search indexer off for Windows Seven.