Basically what this is for is, lets say your remote shell has hung up on you. Normally what I would do is open another shell and kill the process. This is obviously a pain to keep doing.
A quick bit of googling and I found this:
http://www.theclonchs.com/wiki/SSHBreak
You can either edit the global ssh_config as the example in that URL says (/etc/ssh/ssh_config) or in the local user configuration (~/.ssh/config).
So to test that you have it working properly do:
CODE
mark@tone ~ % ssh localhost
Password: <password>
Last login: Fri Jul 20 15:45:45 2007 from localhost
mark@tone ~ %
mark@tone ~ % Connection to localhost closed.
Password: <password>
Last login: Fri Jul 20 15:45:45 2007 from localhost
mark@tone ~ %
mark@tone ~ % Connection to localhost closed.
So I typed Enter ~. and it quit. No more hung ssh connections! YAY