The legend of the Boo Hags originally comes from South Carolina's rich Gullah culture. According to the myth, Boo Hags are similar to vampires, except they steal your breath by "ridin" you. They are also supposedly much more frightening in appearance. An expression sometimes used in South Carolina is "don't let de hag ride ya." This expression obviously comes from the Boo Hag legend.
If you were ever to meet a Boo Hag, you would recognize them because they have no skin at all and are blood red in color. The Boo Hags are flaxen, which makes them appear raw and also makes them hard to hold on to, and their skin is also very warm. Due to their appearance, they tend to disguise themselves in others' skins. As they wear the skin as we might a costume, they freely go about their business to find people to "ride."
For Hags to go ridin', they must take off the skin and hide it so they can use it later. After a victim has been chosen, the Hag will fly to that person's house and gain access by sneaking in a crack. When the Hag starts to ride the victim (while they sleep), that person is rendered helpless. When the Hag is done ridin' it flies off to retrieve its skin. The victim awakes only feeling tired. The Boo Hag doesn't kill the victim because he or she could be a continuous source of energy for them to ride. But if someone started struggling, the Hag would consider them not worth riding again and would take their skin.