He either reacted to the pain or to get out of the way but he didn’t get knocked down from the force of the bullet. The guy in this video takes a round of 7.62x51mm while standing on one leg and barely gets pushed back: https://youtube.com/watch?v=o5f1Fo4r4_I

How much pain the receiver experiences varies depending on the backface deformation of the plate. If there is no deformation, the plate evenly distributes all of the force and it’ll just feel as if someone punched the armor plate. But if there is backface deformation, the force gets concentrated in that spot and can cause anything between bruising to broken bones and internal bleeding depending on the amount of deformation.