From the Tea Girl

"A bamboo matcha whisk, also known as chasen or 茶筅 (tea whisk) is made from hand using a carbon knife to split the bamboo rod into over 80 tines and shaped and curled into an iconic trapezoidal shape.

A matcha whisk is one of the key accessories in matcha preparation and as such, it is vital to have the right kind and quality to make a good matcha. A traditional bamboo chasen is used to mix the Matcha powder with hot water without lumps, and produces a creamy, delicious froth."