The V2.5 release of the SP includes a pair of mechanisms that support integration with authentication mechanisms that are outside the SP itself. You can think of this as the reverse of the approach described here under "Provisioning the Application's Database or Session".
Instead of transitioning from the SP session to something else, the SP can now support transitioning from "something else" into an SP session. The external mechanism can be anything, including local authentication by password, or other SSO protocols. Technically it doesn't even have to live on the same server as the SP, because this mechanism is in principle a form of SSO in its own right.
Obviously, there are security implications to using this feature, and you should understand them well before enabling them.