Set a comfortable 70 BPM. Hear eight beats with subdivision 2, stop, switch to 3, and restart while continuing to tap the same quarter-note pulse internally.
Notice the moving midpoint
The second eighth lands halfway through the beat. In a triplet, the second click arrives earlier and the third later; neither matches that midpoint.
Keep the accent immovable
Judge the transition by whether your main beat remains aligned with every loud click, not by how fast the inner notes feel.
Repeat the change in both directions
Practise two to three, then three back to two. Leave two accented beats of beat-only mode between changes so you can verify the anchor. When both directions stay stable, remove the pause but keep listening for the same loud quarter-note.