Sentence:  Before the contest even started, Slater went down hard in a warmup session. He took a two-wave hold-down in the semifinals, his board <WSD>tombstoning</WSD> eerily for all to see, ... 
Meaning of the word marked with <WSD> tags:  For a surfboard to stand upright half-submerged in the water (like a tombstone, above) because the surfer is underwater with his or her legrope pulled tight. Often this indicates a surfer in difficulty, either held down by the power of a wave or unconscious and unable to get to the surface.