It denies TCP/IP access to registered IP addresses by using the ipfw packet filter.
Originally, the authors conceived an MT Proxy to rate-limit the SMTP connections of messages believed to be spam.
That worked by adding a dummynet rule for frequent senders who had been sending messages that triggered an unreliable statistical analysis.
[1] A key limitation of the original scheme was the consumption of local resources (in the SMTP proxy).
[2] That probability is subject to a decay, configured as a halving time period, so that each IP address is eventually rehabilitated.