The documentation for the [rename] command is available at http://purl.org/tcl/home/man/tcl8.3.2/TclCmd/rename.htm .
The [rename] command is most often used for wrapping commands. In other words, the programmer uses [rename] to hide another command -- so that a wrapper can be placed around the hidden command to change its behavior. Examples of this important concept abound on the Wiki; among the clearest are: DGP: Be aware that if you rename a command into another
Tcl syntax help - Arts and crafts of Tcl-Tk programming - Category Command