Contortionists receive their training from circus people or through some strange religious discipline. The skill allows them to control their bodies in ways not possible for normal people: moving bones out of the sockets, distorting the physique in lots of horrible ways, and otherwise being revolting.
They can escape from ropes, chains, or other bonds on a successful test against Dex with a penalty equal to half the Intelligence of whoever secured the bonds.
Contortionists can also squeeze through ridiculously narrow openings, such as the bars of a cell or a narrow ventilation shaft. 6" x 1' is the narrowest practical gap such a character can squeeze through. Even though many people find their antics repulsive, contortionists have a +10% modifier for all Busk tests.