Why are people so amazed with this?
Kakashi could probably do this since the beginning of part 2. He just used a reversed Kamui.
But what would be the point in using a reversed Kamui back then?
The only strange thing is that it seems like Kakashi never tried absorbing a bunshin with Kamui, just to see what would happen to it. But perhaps he already thought of this, he just didn't know if he could use kamui from the other dimension, if his clone would be too weak to do it.
Now that he knows it is a different story. Cause no one would "test" this on yourself...
The how did he know, where to create a reversed kamui for Bee? Naruto simply tossed all 3 of them. This way they all would have the same speed and direction, so Kakashi could easy figure, where and when to reverse the kamui.
Another possibility is that he just used a normal kamui, and realized that his kamui is actually a 2-way door, which absorbs depending on which side of the door, you are on. And Obitos is a 2 way as well, it's just controllable throughout his body, which part of the world, he wants to be in.