Yes, I believe it is WebHTTrack. Feedcircuit isn't in charge of reading content, it merely downloads the RSS feeds (and stores it for X amount of days). When you click on the specific feed (say Reuters News) it then opens up the default microb browser (though if you have the script to change browsers it will open up the corresponding browser.. e.g. Tear). It is Tear/MicroB/Midori/Whatever that then handles the feed (basically browsing it offline). Basically put, it's like saving a website on your computer.

Hmm I have used ReadItLater but I haven't tried the offline access yet. I'll try it out sometime.