I'm a little late in writing this. Last week we were talking about how many of the swimmers are obviously getting cupping therapy (they look like they are covered in perfectly round hickeys)
There are several different versions of cupping, but at our clinic cupping involves applying round suction cups to a client's skin, and by using a small air pump the skin is pulled away from the underlying muscles. This increases blood flow to the area. Cupping is frequently used to relieve sore muscles.
With these olympic swimmers, it sounds as if cupping is being used to help muscles recover more quickly after a race. Anyway, you can read more with this global news article: