49 CREW POSITION; LIFELINES
49.1 Competitors shall use no device designed to position their bodies outboard, other than hiking straps and stiffeners worn under the thighs.
49.2 When lifelines are required by the class rules or any other rule, competitors shall not position any part of their torsos outside them, except briefly to perform a necessary task. On boats equipped with upper and lower lifelines, a competitor sitting on the deck facing outboard with his waist inside the lower lifeline may have the upper part of his body outside the upper lifeline. Unless a class rule or any other rule specifies a maximum deflection, lifelines shall be taut. If the class rules do not specify the material or minimum diameter of lifelines, they shall comply with the corresponding specifications in the World Sailing Offshore Special Regulations.
Note: Those regulations are available at the World Sailing website.