Say you have a machine named MYHOST want to bind your port #777 to the TELNET port of the host REMOTEHOST.
Use: proxyd 777 REMOTEHOST telnet
Then if you did 'telnet MYHOST 777' you would actually connect to the TELNET port of the host REMOTEHOST.
As far as the 'service-name/port-number' goes: services can be referred to either by a number or a name. For example, NNTP requests are usually on port 119, so if you wanted to connect directly to a machine to do NNTP you could do either:
telnet REMOTEHOST nntp
or
telnet REMOTEHOST 119
NOTE/WARNING: Make sure to use an unused port on your machine! If you are not sure, do this:
nidump services .
to see a listing of all the services known on your machine.
This program does NOT always have to be run as root... Some ports are available even to regular users, it appears.