Search Appliance
After a successful walk, Results Authorization is configured with the Results Authorization Options group on the Search Settings page. The primary setting is Authorization Method (here), which is determined by the authentication system(s) in use by the indexed URLs. If cookie-based, this is set to Forward login cookies; for all other systems, it is set to Basic/NTLM/file - Prompt via form. Most of the remaining settings depend on which method was selected; see the Authorization Method setting (here) for details.
There are also a few resource/tuning settings, such as Max Docs to Auth-Check, Successful Auth Result Limit, and Total Auth Timeout, which are not required, but merely fine-tune the results.