Notice that they are not saying, because of the sound of the bell, but in the very sound of the bell itself, all great Zen Masters attain enlightenment. At the very sound of the drum, they fall down. The falling down of falling down. The enlightenment of falling down. From one point of view, they originate from the same source, from no sound at all.
Sound arises from no sound.
From one perspective, the sound of the bell and the sound of the drum are exactly the same. From another perspective the sound of the bell and the sound of the drum are quite different. Enlightenment and delusion. And yet, Zen Master Seung Sahn says if you hear either you fall into hell. Our life is falling down, getting up.