There are two types of cupping, wet and dry. Dry cupping involves the placement of small glass or bamboo jars all over the intended area, like the back, or a knee, for instance. Wet cupping involves the same thing, but with a small incision made for each cup first.
The cups are suctioned so that they pull on the skin and the blood, drawing it to the surface. This is said to help blood flow more easily in places where it doesn’t typically reach all that well. It’s also said to help break up blockages of debris within your body, as well as congested energy.
As far as muscle growth goes, the increased blood flow to broken down muscles is what helps them grow back much more quickly. Similar outcomes are desired by the makers of items like leg boots, which use pressure to force blood into the legs to help them recover faster.