A Person Is Running a Potter's Wheel, Where They Use

A person is running a potter's wheel, where they use their feet to run pedals that turn a large disc they can put clay on. Suppose the disc is turning and some clay is dropped straight down on the rotating wheel, which slows down a little. Why did the wheel slow down when the clay was dropped on it, and where on the wheel should the clay be placed so that it is easiest for the potter to speed the wheel back up again? Please explain.