Live concert performance.
The point around 2:12, you can hear the choir gets out of time, the baritones mess it up, Callas hears it, you see her half turn, she raises her right hand and...signals down and sings her note and BAM everyone is back in time. Unbelieavble. That's the mark of a true master, she corrected the Orchestra, the Conductor and the Choir with one gesture. Very, very few singers can do that. It shows complete mastery of the piece and the arrangement.