This is the crux of the whole Genjokoan. He’s saying that when you look at your reflection in water, there’s a tipping point: either the water’s transparent or it’s murky and reflective like a mirror. It doesn’t gradually become a mirror. It’s either a mirror or it isn’t. Dogen is using this as a metaphor: when you drop off the body-mind, when you forget the self, you become a mirror that reflects the dharma.