He’s talking about emphasizing the off-beat, or the back-beat, which you can do easily enough on the fiddle.
But it can be done on the whistle too, by means of phrasing and yes volume, or pulsing of the breath. Ornaments such as rolls can help too, if you play them in a certain way.
However your man should have said the 3rd and 7th eighth notes, not 3rd and 6th. This corresponds to the 2nd and 4th beats of the bar.
Two easy examples that spring to mind:
The first bar of The Earl’s Chair could be played B2 dB {c}BAFA
You can easily make the high d jump out by slurring into it and giving a little push to the breath as you slap your fingers down. The same for the F# later in the bar.
The first bar and a bit of The Lady on the Island could be played BAFB AFEF | D2 FA …
You can play this to make the first F# and the E jump out.
But: the phrasing of this particular tune demands that the first beat of bar 2 gets an extra wallop.
Which brings me to a big caveat: as someone who played Irish music on fiddle and whistle for getting on for 30 years, I would strongly advise you to avoid applying play-by-numbers formulae such as “emphasize the third and seventh note of the bar”.
If you do this systematically your tunes will soon sound horribly monotonous. Just like a lot of modern fiddle players, in fact, who apply “bowing patterns” too rigidly. They sound like rhythm machines rather than people playing a melody. Reels are tunes, after all.
Each tune will require its own treatment. If you applied the formula, you’d miss the big downbeat in bar 2 of Lady on the Island, which would be to miss the essence of the tune altogether.
To get the hang of the backbeat idea, I’d suggest you listen to recorded fiddle players and notice how they bring out the backbeat in a particular tune. It won’t be every 3rd and 6th note, and not in the same way. Then let the whole idea percolate into your subconscious. When it’s ready to come out in your whistle playing, it just will, easily.
If you still want it to, that is. You can play fine traditional reels on the whistle without making too much of this backbeat issue anyway.
HTH
Steve
PS Bodhran players need to be keenly aware of the backbeat, I would think. More than whistle players, in fact.