The basic rule is that the crew can trim sails and hull and do normal seamanship, but cannot move their bodies to propel the boat. Five actions are specifically prohibited: pumping (repeated fanning of a sail), rocking (repeated rolling from body movement, sail or board adjustment, or steering), ooching (a sudden forward body movement stopped abruptly), sculling (forceful or propelling repeated helm movement), and repeated tacks or gybes unrelated to wind shifts or tactics.
The exceptions matter as much. You may roll the boat to help steering, and roll-tack or roll-gybe as long as you do not come out faster than you went in. When surfing or planing is possible you may pull each sail in once per wave or gust to start surfing or planing, and to start foiling you may pull in as often as needed. You may scull to turn to close-hauled when stationary or slow above close-hauled, pump to pop an inverted batten (if it does not clearly propel), slow down by moving the helm repeatedly, and use any means to help someone in danger or to get clear after grounding or a collision. Sailing instructions may permit engines in stated circumstances.
World Sailing publishes official interpretations of rule 42, and many classes relax the rule above a set wind speed (Appendix P, flag O).
42.1 Basic Rule
Wind and water only; normal trimming and seamanship allowed.
42.2 Prohibited Actions
Pumping, rocking, ooching, sculling, unnecessary repeated tacks or gybes.
42.3 Exceptions
Steering rolls, roll tacks and gybes, the once-per-wave pump to surf or plane, pumping to foil, sculling to close-hauled, batten flicks, slowing down, rescue, getting clear, and engines where the sailing instructions allow.
Changed in 2025
The foiling exception is explicit: to initiate foiling a sail may be pulled in any number of times.