I think it's talking about what I call the "flip-flop" bass pattern when I explain it to my students.
The key to the whole thing is the idea, rooted in Western Classical tradition, that the strongest bass or harmonic movement is down a fifth. For us accordionists, that's also "toward the floor". I like to think of gravity pulling chords and bass notes
down toward whatever key the piece is in.
Normally, when you play the standard alternating bass pattern, you play the
root of the chord first, then the chord, then the
fifth of the chord (the "upstairs" bass note). The overall pattern is root, fifth, root, fifth...
But when the chord in question is the V7 chord (the chord that's "upstairs" from whatever key you're in at the moment), it's common to
flip that pattern around, so that you
start with the fifth. That is, fifth, root, fifth, root...
If you try playing a bar each of C, G7, then C again, trying both ways, you'll see the reasoning behind it. Without the flip-flop, the G bass repeats when moving from the C to the G7. Worse, when you move back from the G7 to the C, the bass motion is a boring old D to C movement. A resolution of a mere whole step (or major second). Big deal.
But with the flip-flop, there are no repeated bass notes in a row,
and that final bass motion, when you resolve the G7 back down to the C, is a cascade of nice, strong, "down a fifth" movements: D to G to C. Ah! Much better. Gravity at work!