The minor 6th and major 6th that Fintan plays are in tune. Listen for the combination tones - a lower-pitched note created by the difference in the frequencies of the two notes in each chord. There is a clear change in the combination tones between these two chords.
Combination tones (also called difference tones, Tartini or third tones) occur when a double-stop is in tune. The chord played here is A on the E string with C# then with C natural on the A string.
These combination tones are easier for the player to hear than the listener. The audience will appreciate the increased resonance.
When you play a chord and listen for the difference tone, you may at first hear a 'sympathetic buzz' or increased resonance rather than a clear lower-pitched note, but the search will improve your intonation and resonance.
Resources - Bibliography
- for The Art of Tone Production by Sherman Pitluck
Scores - Combination Tone - violin & viola