Balance on the dome and rotate side to side.
Turn from your ribcage, not just your arms, and let the wobble force your core to stabilize.
Sitting on the dome in a V position with feet floating means your core is balancing before the first twist even starts, and each rotation shifts your weight across a surface that gives underneath you. The obliques drive the turn from side to side while the deep core stops you rolling off. It earns its place as rotation training with a built-in stability tax that the floor version simply does not charge.
On the floor your sit bones are anchored, on the dome they are not. Every rotation shifts your balance point, so the core works between reps as well as during them, and sloppy momentum gets punished immediately.
Up, off the floor, as the setup describes. Feet down makes it dramatically easier and is a fair regression while you learn to balance on the dome, but the full version is feet floating.
You are probably twisting your arms while your torso stays still. Rotate from the ribcage so your chest actually turns each way, and lean back enough that your abs, not just your hips, hold the V.
