Uncapping, in the context of cable modems, refers to a number of activities performed to alter one's 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 may be considered an illegal activity and many internet service providers check modem configuration files nightly to detect uncapped modems. If caught, uncappers risk possible account termination and/or prosecution for theft of service. Note that regardless of legality, uncapping may be a violation of the Terms of Service agreement that the customer has with the ISP.
There are several methods used to uncap a cable modem, by hardware,software,tricks,alterations, and modifications.
The first technique (there were others before this) to enjoy any success employed Motorola modems (such as SB3100, SB4100, and SB4200 models). By spoofing the TFTP server, one could force the modem to accept a different configuration file than would be expected. The configuration file is a file stored on the TFTP server that tells your modem the download and upload caps for it. An example of spoofing would be to edit the configuration file (Requires a DOCSIS editor) or replacing your configuration file with one from a faster modem (Through Gnutella Networks).
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).
Another more advanced method is to attach a TTL to RS232 adapter, and get access to the console directly thus making the modem download new firmware, which can then be configured via a simple web interface. Examples include SIGMA (SIGMA is a Firmware Add-On that expands the current features of an underlying firmware.) and others. Refer to the sites below for more information and tutorials on various methods used.