The idea behind Core Ball fits into one sentence: push needles into a spinning circle without letting two of them touch. That is why it spread so fast. You understand it in five seconds, and it still manages to be difficult for a long time after that.
You play with a single click or tap. Each press fires a numbered pin from the bottom, and it sticks into the rotating wheel wherever the circle happens to be at that moment. If the new pin lands on top of one that is already there, the level is lost. The number in the middle of the wheel shows which level you are on, and every new level makes the job harder.
The trick is rhythm. Do not shoot as fast as you can; watch the wheel for a few turns, find the gaps and fire when one passes the bottom. It runs in the browser on a computer or a phone, with nothing to install. Timing games such as Geometry Lite train the same skill.