What are closed qigong exercises?
The attention is inside. You work with the qi available in the body. This requires a healthy body and sufficient qi.
During the exercise you guide qi in the meridians. If you lose concentration, this can result in misguiding with obstructions or blockings of qi.
When you reach a certain level (Gongfu), you then open yourself to the outside. This opening to the outside can have very strong side effects.
In both cases you need the help of a master.