The difference between a real Russian twist and a fidget is where the rotation happens. It should come from your ribcage twisting against locked hips — hips and feet stay still, knees stay together, only your torso turns side to side. Sit with your knees bent, torso leaning back to about 45 degrees, chest up so your spine stays long rather than rounded. Touch the floor lightly beside your hip on each side, don't slam it. Keep your feet up for the harder version, or keep your heels down if you're new to it — both are fine, the rotation is what matters, not whether your feet are floating.