It is a module that sends static content at a bandwidth rate limit, defined by the various old modem standards. So, you can browse your site with a 56k V.92 modem, by adding something like this:

Previously to do bandwidth rate limiting modules would have to block an entire thread, for each client, and insert sleeps to slow the bandwidth down. Using the new suspend feature, a handler can get callback N milliseconds in the future, and it will be invoked by the Event MPM on a different thread, once the timer hits. From there the handler can continue to send data to the client.