Each tclhttpd process is configured by a configuration file, usually 
designated with the -config command line option.  The virtual host 
patch creates another configurable option to appear in that file:

Config virtual [list $hostNameList1 $configFile1 \
		     $hostNameList2 $configFile2 ...]

A slave interp will be created for each $hostNameList, and in that slave 
another complete copy of the tclhttpd packages will be loaded according 
to the configuration specified in the corresponding $configFile.  All
the host names in the $hostNameList will be associated with that slave
interp; a request for any host in the $hostNameList will be handled by
the corresponding slave interp.  

An example configuration line might be:

Config virtual	{ dev.example.com 		dev.rc 
		 {example.com www.example.com}	users.rc
		}

Here example.com and www.example.com are multiple names for "the same"
site, and that sameness includes sharing the Tcl interp that handles
requests.  This can be important depending on how the site makes use
of the interp.

From the perspective of the creator of the virtual site, nothing changes, 
they still create a $configFile to set up their site and interface it 
with whatever components they are using to build their site.  Their 
interface code is still running in a Tcl interp; the fact that it's a 
slave interp makes no difference.

The actual mechanism is that the master interp accepts all socket 
connections.  When the Host: header on the request is received, the 
connection gets passed to the appropriate slave interp for further 
handling.  If there is no Host: header (really ancient clients?) or if 
the Host: value is not one of those specified in the [Config virtual] 
argument, then the master interp handles the request according to 
whatever additional configuration details are found in the master 
configuration file.  In a sense, every IP address served by tclhttpd 
has a "default" site determined by the master config file.

The configuration values used in the slave interp are first inherited 
from the master.  In the $configFile, you only need to specify those 
values that distinguish the virtual site from the default site.  In my 
experience that usually includes the following [Config] options: 
docRoot, main, webmaster, and LogFile.