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.