It's a difficult technique to pull off without tipping the audience off, particularly in film. A director often has no choice but to cut to a close-up of an object in order to show it. An example of how not to do it is in John Carpenter's Halloween, when a nurse lights a cigarette and - for no other reason that it will later provide a clue - we are given a ten second Panavision close up of a red matchbook. Far subtler an example of foreshadowing can be found in the opening few minutes of Roman Polanksi's Chinatown, when fisherman Curly makes a barely audible reference to 'albacore' in his exchange with Jake Gittes, a word that ultimately helps the detective crack the Mulwray case.
In fact, as the above two examples show, it's easier to smuggle things through in written rather than visual form. Benchley does it with Quint's knife ("He stabbed the knife into the gunwale, freeing his left hand to hold the rope, his right hand to shove the barrel on deck.") in his busy description of the fisherman snagging one of the barrels. Just over a page later, the significance of the object is revealed ("The knife was there, embedded in the wood."). In the movie - where the knife has morphed into an Indiana Jones-style machete - Spielberg inserts a close up of the blade embedded in the wood. Its obviousness is made even greater by the continuity mismatch of the background ocean, which is smooth and almost golden in the sunlight.
In the end, it's a wasted shot, anyway. When Quint struggles in the jaws of the shark he has the machete in his hand, but we are given no explanation as to how he managed to grab it from the gunwale. Although the audience are so caught up in the action they don't really care, the close-up of the machete remains a minor flaw. To be fair on Spielberg, the director handles the business with the air tank with an expert sleight-of-hand. Had Chekov seen the movie, he would have approved, and might have remarked, "One must not put a compressed air tank on the boat if no one is thinking of blowing it up."
