The SPS Help includes the following topics on secure testing:
Testing assets before making them public: http://help.adobe.com/en_US/scene7/using/WSd968ca97bf00cf72-5eeee3a113268dc80f5-8000.html
How Secure Testing works: http://help.adobe.com/en_US/scene7/using/WSd968ca97bf01df72-5efde3a123268dd80f5-8000.html
Testing the Secure Testing service: http://help.adobe.com/en_US/scene7/using/WSd968ca97bf01cf72-5cede3a113268dc80f5-8000.html
We also have a 2011 white paper created before Secure Test was released.
You can find it below: