Definitions for "Fall off"
Keywords:  farther, errant, leeward, jump, wind
come off; "This button had fallen off"
fall heavily or suddenly; decline markedly; "The real estate market fell off"
This doesn't mean jumping off your board. It means to change direction so as to point beyond away from where the wind is coming from.
To cause conscious crew members to become frantic and yell "Man overboard".
Keywords:  traitor, rebel, henry, king, part
become a rebel or traitor. (King Henry IV, Part 1)
Keywords:  diminish, intensity, size
diminish in size or intensity
Keywords:  head, down, see
See Head Down.