Auto-Key is an Antares plug-in that can analyze polyphonic (multi-voice) audio and determine its key (C Major, G Minor, etc.) For this reason, we highly recommend that you do not load Auto-Key as a plug-in insert on any vocal tracks you're planning to process with Auto-Tune.
You should instead add Auto-Key as an insert on instrumental tracks that determine the overall key of your song. For example, if you're producing a song with one vocal track and one guitar track, and are planning to pitch correct the vocal track with Auto-Tune, it would be better to load Auto-Key on the guitar track instead of the vocal track. The guitar track would likely contain chords which determine the overall key of the song, whereas the vocal track would only contain a single voice melody that may be out of tune and not accurately reflect the key of your song.
If you're not exactly sure which audio tracks in your song contain chords that determine its key, you can simply load Auto-Key on the master output bus of your project's mix to have it analyze the output of all the audio tracks in your song.
When using Auto-Key, we recommend a Hardware Buffer size of 512 samples and when done using the plug-in, please bypass or remove Auto-Key to free up your computer's processing power.