The third leg of the race was the most important part of the relay race in the 4x100m competition. Compared to the other legs, the third runner had to run the longest curve, and had to complete the final handoff, which required the best technique.
In addition, the third runner tended to feel nervous because the upcoming curve was the final one. They wanted to hand off the baton as soon as possible. However, the more nervous they were, the more mistakes they tended to make.
The fourth runner, on the other hand, only had to focus on one thing once they got the baton in their hand—running! This was the only thought in their mind, and because of this they generally felt less nervous.
Dai Li had chosen Kenen Shen to be the third runner because he was good at running the curve and had a lot of experience.