The vacuum exercise is an exercise which involves contracting some internal abdominal muscles, primarily the Transversus abdominis muscle, and not as much the diaphragm, the "six pack" muscles or "abs" which are trained through crunches, leg raises, or other core exercises.
Repetitions of the exercise may be used as a form of endurance training, and light strength training. There is difficulty building strength in the muscle, as it is not easy to apply resistance training to the deeper internal muscles.
The reasons for performing this exercise vary. It has been done for aesthetic purposes in bodybuilding competitions (to suck the abdomen in, making it appear less bulgy). It can be done to enhance overall core stability and strength. It is used in belly dance to actively perform flutters, engaging various fibres in the muscle selectively. Some believe the pressures it exerts upon the intestines are an aid to digestion. It is used in reverse breathing upon exhalation.
People may also sometimes contract these muscles in public to reduce the appearance of their abdomen, consciously or unconsciously. "Sucking in" the stomach to appear thinner may be most common form of this exercise, but with little of the intensity or long-term purpose of the other forms.
Other articles related to "vacuum exercise, exercises":
... Hypopressive exercises also involve transverse abdominis, but they are based upon reflex contraction of the abdominal wall, rather than voluntary contractions that are a feature of ... of this is thought to be that hypopressive exercises increase the base tone of the abdominal wall (as well as the pelvic floor) and hence reduce resting waist circumference ...