Knowing its coming and being ready for it lets you keep your balance even if you are standing on one leg.
Heres a U S soldier who did not know it was coming and his vest which is far more bulky than the one in the other video stopped the bullet, yet he ended up on the ground.
http://www.youtube.com/watch?v=bQi7iknSKy0
He recovered quickly but he did go down.
There are other similar clips, With the same pattern of being knocked down by a bullet strike, but with less information on whether the soldiers survived or where the bullet struck, so I don't feel comfortable linking to those.