A chasen is a bamboo whisk used in mixing Japanese powdered tea in a chawan (tea bowl.)
It has very fine tines attached to a wooden handle. At the bottom of the whisk, the tines don’t meet, the way they would with most normal kitchen whisks. Instead, they are just curled up at their ends, leaving the bottom open.
Chasen with thinner tines, called Kazuho type, are used for thin teas (“usucha“.) Ones with thicker tines are used for thicker teas (“koicha”.)
Better-quality chasen will have about 120 tines on them, and be made in Japan; lesser-quality ones will have about 60 tines, and usually be made in China. The better ones have the tines curled by hand, and are carved from a single bamboo piece. The cheaper ones are machine-shaped, and may have the tines go straight after a few uses.
You don’t actually “whisk” the tea with them the way you would egg whites; you move it in a “m” motion instead in the tea bowl.
To clean, rinse then dry before storing. You can buy stands (referred to as “forms”) that you put them on to dry to help them retain their shape longer.
Old and broken ones are taken to temples once a year, generally around May, and burnt there in a ceremony called “chasen koyō.” It is considered bad form to simply toss one in the garbage,
For guests, only a new one should be used.