Complex sentence: The vacuum exercise is an exercise which involves contracting some internal abdominal muscles, but not the diaphragm, the "six pack" muscles or "abs" which are trained through crunches or leg raises.
Split into simpler sentences: The vacuum exercise is an exercise which involves contracting some internal abdominal muscles. This is not the diaphragm, not the abs referred to in the phrase six pack, and is not trained through crunches or leg raises.