Essentially, Herbert Hoover fell slightly differently when he was pushed to the ground by another child when he attended school. His spine hit a rock just right and he developed a permanent limp. Hoover walks far more slowly than in the original timeline (OTL).
But what does this affect?
During his and his wife's stay in China during the boxer rebellion, the Hoovers assisted American forces in navigating Tianjin.
Herbert, specifically, guided marines through the streets. He was, of course, slower at this.
Fed up with his pace, the marines declined his help.
Instead they attempted to use locals for direction.
However, the locals sympathized with the Yihequan and often lead the marines into traps.
As the Yihequan (in collaboration with the Qing Imperial Army) gained control of Tianjin, they gained control of American imported arms.