Kensui are waste water containers used in the tea room during the preparation of matcha. They are often made of copper or brass, but can also be formed in ceramics, wood or lacquered wood. One of the most difficult moves in the tea room, is standing-up after preparing tea on the tatami mats. You stand up while holding the kensui, often nearly full of water.
Tradition dictates that this is the one time the host turns away from the guests, instead of towards them. I have read that this is to avoid the kensui passing over the fire! If the used water from the kensui were to spill on the brazier (your feet are probably still asleep at this time) the resulting steam and ash would be explosive. So, turn the opposite way in consideration of the guests.