The yellow line is there in make sure that no one hands or sneaks the bullet from one side to the other. If on one crosses the line it is only possible for the bullets to get from one side to the other by being shot across the line.Nice trick like most of what they do. I also think it has something to do with the yellow line. Those are very mild loads for a 357.
Very close but not exactly rightThe way I would do it is to actually shoot the bullets with a very mild load, but shoot so they would hit the vest and the vests might be magic props. Have a bullet in my mouth and make the switch with slight of hand with the one that hit the vest. At least something along those line and you would have to practice a lot.