It advertise that it can download files 400% faster
Maybe if you are on a 2400 baud modem. It probably loaded some adware/spyware to your system as well.
The way so-called download acceleratores work is through opening multiple connections to the HTTP server which allows it to download multiple ranges at the same time. Once in a blue moon, when the site that the file is being downloaded from is artificially capping download speeds, a download accelerator will actually work. It will work by bypassing limits which were probably put in place for a very good reason, but it'll work. This is bad, because HTTP servers can only handle a finite number of simultaneous connections, and so when one person is opening up four or eight or twenty connections, he's filling up spaces that could otherwise be used to allow that many other people to connect to the server. Get enough people doing this, and soon no one will be able to connect to the server.
Rule #1 for everything: If it sounds to good to be tue, it probably is.