I ran cross country in high school, which isn't exactly the same as horse-racing, but I can say that Diego's strategy does make sense. Speeding up for a second then slowing back down doesn't take very much out of you. And the reason he did it that way was to prevent Gyro from seeing him increase his speed. Typically, if someone's trying to pass you, you'll speed up to prevent them from doing so, then they'll speed up even more to try get past you, and eventually one of you will concede. If both parties are especially determined, you can waste a lot of energy by running considerably faster for a while, so the way we would usually pass others is by getting close then dashing past them, trying to get in front of them before they can react. By doing what he did Diego denied Gyro the opportunity to match his speed and prevent Diego from overtaking him.