Pen control training is a writing exercise. Writing here mainly refers to brush calligraphy, but some hard-pen calligraphy also has pen control training.
The so-called pen control can be simply understood as "controlling the writing brush". Because the brush is soft and needs to be controlled.
A writing brush is a kind of pen made of animal hair, which is very soft. It is precisely because the brush is soft that it is difficult to control. So it is necessary to control the brush.