Most Southeast Asians beliefs in health and medicine stems from the Chinese concept yin and yang. They believe everything in life is held at a balance and illnesses are something that happen when things are out of balance.
Cao Gio is also called "coining" which is when a person combines various oils with the rubbing of a coin along the ribs or spine. Cao Gio is supposed to be a method of relieving ailments such as fever, nausea, colds, and pretty much any illness that could be related to the weather. The reason for it's relief of weather related illnesses comes from their belief system. Cao Gio is a translation of "catch the wind." Southeast Asians believe that these certain illnesses occur when your body absorbs too much wind and the rubbing of the coin until blood reaches the surface of the skin is their way of "releasing" the wind and restoring balance.
While only Southeast Asians have given testimony to this method being successful, it is still a practice that is commonly used today and has even been used in the U.S. by people of Southeast Asian descent.
Here are some pictures of Cao Gio: