A dive with good hand extension like de Oliveira's is a great example because it's not the hands that dictate when the racer officially finished. The competitor is said to have finished when their torso moves across the finish line. That's why racers lean forward at the end of a race and don't stick out their hands to break the plane.
The rule from the IAAF reads: "The athletes shall be placed in the order in which any part of their bodies (i.e. torso, as distinguished from the head, neck, arms, legs, hands or feet) reaches the vertical plane of the nearer edge of the finish line as defined above."
It's not something you see often, but as Olympic hurdler and bobsledder Lolo Jones said on Twitter, "most pro's [sic] at one point have used that tactic to win." Fans of Felix might not love it, but diving is admissible even when the sport isn't diving. It's a disappointing end for Felix, but with her seventh Olympic medal she's the most decorated female in Olympic track and field history.