Sentence:  As you encounter waves on the way out, you have five ways of getting past them: ... <WSD>Turn turtle</WSD>. Turn upside down and hang onto the "rails" (i.e., the edges) near the nose of the board, pulling it downward, so that the oncoming wave pushes on this slanted shield, forcing you and the forward part of the board downwardnb.... 
Meaning of the word marked with <WSD> tags:  To roll upside down with one's surfboard (usually a longboard) to allow a wave, especially a wave that has already broken, to pass over.