Uncapping
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Uncapping, in the context of cable modems, refers to a number of activities performed in violation of one's proper internet service provider modem settings. It is sometimes done for the sake of bandwidth (i.e. by buying a 512kbit/s access modem and then altering it to 10Mbit/s), pluggable interfaces (as by using more than one public ID), or any configurable options a DOCSIS modem can offer.
Uncapping is obviously an undesirable practice for the ISP, though it is notoriously difficult to prevent. Traditionally, ISPs would merely terminate the accounts of those found to be uncapping their modems. More recently, ISPs have begun regularly prosecuting uncappers under theft of service laws.
Uncappers are almost always caught because cable internet providers generally check the configuration files from the head-end nightly.
Methods
There are several methods used to uncap a cable modem, by hardware or software tricks.
The first technique to enjoy any success employed Motorola modems (such as SB3100, SB4100 and some of the SB4200 models). By spoofing the TFTP server, one could force the modem to accept a different config file than would be expected.
An alternate method employs dhcpforce. By flooding a modem with faked DHCP packets (which contain configuration filename, TFTP IP, etc), one can convince the modem to accept any desired config file, even on one's own server (provided the server is routed, of course).