The "sso post" feature is an alternative to using SSO with NTLM and BASIC Http authentication
http 401 code response), or use http header X-SSO-USER to pass the username/password.
The "sso post" is design for customers who do not use NTLM or Basic Http Authentication method
on their web application. These are customers who have their own web authentication through "form method".

The administrator will configure the URL and username/password "POST fields" for the application login page.
If the SP receives a response to a request that matches the specified URL, it will look for the specified
names in the form's POST fields. If these fields are found, the SP will populate the field with the sso
username and password and generate a POST to the authentication Web Server.

Here are the links to the PowerPoint and PDF that contain detail explanation and instruction to setup SSO post in general and for OWA 2007 specific.

http://supportkb.arraynetworks.net/kbcontent/SSO/SSO_POST_HOWTO.ppt

http://supportkb.arraynetworks.net/kbcontent/SSO/SSO_POST_HOWTO.pdf