Racers follow the stream -- jumping off small cliffs and through waterfalls -- and a final rock face at the base creates one of the last challenges, requiring concentration and crab-walking to climb down.

BBC: An Alaskan-style Fourth of July

Aly Raisman didn't use one in Thursday's all-around final, which her coach, Mihai Brestyan, called a strategy to sharpen her concentration.

WSJ: Social Media