Plank meets anti-rotation. Your core hates it.
Keep your hips dead still. If they wobble side to side, widen your feet and slow down.
Every time a hand leaves the floor, one shoulder briefly holds your entire upper body while your core fights the twisting force trying to rotate your hips. That anti-rotation demand is the real product here, the delts stabilise, the trunk refuses to move, and the slower you go the harder it gets. It is a plank that punishes impatience.
Your base is too narrow or your tempo is too fast. Widen your feet, brace before each tap, and move your hand slowly. If a water bottle on your lower back would fall off, the rep did not count.
The shoulders stabilise your bodyweight one arm at a time, while your core does anti-rotation work to keep your hips square. It is less about the tap and more about everything that refuses to move during it.
Easier, widen your feet or do them with hands on a bench. Harder, bring your feet together, slow each tap to a two-second hold, or stack them after push-ups when your shoulders are already cooked.
