Why the shape matters and why it is fragile
A chasen is cut from a single piece of bamboo into as many as a hundred and twenty tines, split at their tips and curled inward. Those tines have to be flexible enough to break clumps without scraping the bowl and numerous enough to aerate the liquid. All of that depends on the tines staying separated and holding their curve, and bamboo dries into whatever shape it is left in. That single property explains nearly every piece of care advice attached to the object.
Before the first use, and before every use
Soak the tines in warm water for a minute or two before whisking. This softens the bamboo so the outer tines flex rather than snap, and it is the difference between a whisk that lasts a season and one that sheds tines in a fortnight. Warm, not boiling: hot water is harsher on bamboo than it needs to be and takes the finish off the tines.
After use
Rinse in warm water immediately, working the tines gently with the water rather than rubbing them. Never use detergent, which bamboo takes up and returns to the next bowl. Never soak it for a long period. Never put it in a dishwasher, which will finish it in one cycle. And never leave it standing in the bowl with matcha residue drying on it, which sets hard between the tines and cannot be got out without breaking them.
Drying, which is the whole thing
Stand the whisk tines-upward in open air, or on a ridged ceramic stand that holds the tines spread apart. Both work; the stand works better and holds the outward curl over a longer life. What does not work is lying it flat in a drawer, standing it handle-up so water runs into the binding, or putting it away in a closed box, which is how mould gets into the binding thread where you cannot reach it.
When to replace it
Tines break. This is normal and expected, and a whisk that has lost a handful still works. Replace it when it has lost enough tines that the foam goes coarse, when the remaining tines have straightened out and no longer reach the bowl’s curve, or when there is any dark discolouration in the binding at the base — that is mould, it is inside the knot, and no cleaning reaches it. A chasen is a consumable and treating it as an heirloom is how people end up whisking with a mouldy one.