The premise was that Internet access was often slow -- but that you typically use an email address issued by your ISP and that your email server is very close to you netwise, so that the download speed between the email server and your computer is limited essentially only by your dial-up connection, but not by congestion, hop times, overloaded file servers, or other "issues" of the global Internet.
So the idea was that instead of downloading files yourself, you'd ask the Downloadslave to download them for you; when it finished, it would send the file to you as an attachment to an email message. Then, you could download the file at full speed by checking your email.
I don't remember how payment worked; I think you didn't have to pay up-front, so perhaps they added ads to your email messages or something.
Also, now that I think about it, downloading the files as email attachments would probably inflate their size by 33% due to the Base64 encoding... I wonder whether that made the download so much faster, then. (The connection to your ISP's mail server would have to be at least 33% faster than to the remote file server for there to be an improvement.)