UPDATE | 6/5 7:40 PM: Comcast objected to the first version of this post, which referred to its method for identifying heavy users as a blacklist. After several conversations, including a call with Mitch Bowling, Comcast’s senior vice president and general manger of its Internet service, I’ve rewritten the headline and a bit of the explanation about how the system being tested works.

Some Comcast customers who actively download software and video files may soon find one set of unexplained delays replaced with a different sort of equally cryptic slowdowns.

Comcast is starting to test new approaches to protecting its network from what it describes as congestion caused by a handful of customers who use far far more bandwidth than everyone else. Until now, Comcast has been using devices that interfered with the BitTorrent protocol—the most common method for downloading large files from computers of other users. BitTorrent is often used by people exchanging pornography and illegal copies of movies, but creators of video and software also choose to use BitTorrent as an inexpensive way to distribute their creations.

[This section has been updated] Comcast will test new devices that can identify which Internet customers have been particularly heavy users of bandwidth at any given time. At times when the network in a given neighborhood is congested, it will slow down the Internet connections of those heavy users.

For now, these restrictions are just as mysterious as the secret blocking of BitTorrent. The company won’t say how much usage will be required to have your connection impaired or how much slower it will get.

Mitch Bowling, Comcast’s senior vice president and general manger of its Internet service, said the system will look at the bandwidth used over a period of minutes. That means that someone who downloads a single song won’t go into the slow late, but someone that downloaded a whole movie might, regardless of whether the movie is from iTunes or over BitTorrent.

Most significantly, Comcast won’t even tell its customers if or when they are having their connections throttled. Mr. Bowling said that customers certainly would notice a slowdown.

If users stop downloading for several minutes—he wouldn’t say how long—their connections will be restored to full speed.

Comcast, which was criticized for not being forthright about its restrictions on BitTorrent, has promised to find a new approach that will block heavy users of bandwidth regardless of what content or communication protocol they are using.

It is starting three 30-day tests, each of a different sort of hardware that it might use as the traffic cop for its new restrictions. On Thursday, it will start tests in Chambersburg, Penn., and Warrenton, Va. Later in the summer it will conduct another test in Colorado Springs, Colo.

Charlie Douglas, a Comcast spokesman, said that the test is meant to pick hardware vendor and test different configurations of rules. He points out that the vast majority of users won’t see any changes to their service at all.

“When we roll this out nationally by the end of the year, all those types of questions will be answered,” he said. “We are trying to figure out what do customers want, what techniques need to be in place to create the best user experience.”

Still, how the company can test the effect on its customers without explaining the rules to them and giving them visibility into their own usage remains to be seen. Until that happens, I suspect Comcast will stay on the black list of at least the heaviest users.