Actually, when Cheng Lian began using the crossbow mechanisms to shoot the strong oxen pulling the chariots, Yizhong realized something was amiss. However, once the war chariots start racing, it's easy for them to speed up, but not so easy to stop them. In this period where oxen pulling carts aren't controlled by nose rings but by the skills of the cattle herders, slowing down is very challenging, at least not something that can be done swiftly.
Therefore, upon discovering the Han Army had crossbow mechanisms with exceptional power and an astonishingly fast stringing speed, Yizhong developed the thought of retreating, since he had already successfully destroyed half of the Han Army's front camp, which was already enough to boast about.
Unfortunately, the galloping war chariots can't stop just because you say so, so Yizhong had to order a gradual slowdown while preparing to charge out from the sides after deceleration.
