Here is a better video.

https://www.youtube.com/watch?v=bFxDNwPsIi0

You can definitely see both boxes shift once the curtain falls and they shift in such a way that it appears weight is being distributed from one end of the box to the middle.

here is the top youtube comment that explains it: Wheel slowed until controlled speed to land on black. Twins and hidden floor for the trick. You can see the black mat/floor raised to one side with a lump in the original container. Black also being the color of the shirts to help hide imperfections. None of the volunteers selected speak English or speak at all. When entering the container they all walk feet spread apart to not fall into the pit below. They put their hands on the side of the wall when entering to brace support for not falling inside way before the magician instructs them too. The male volunteers both have a bandage on their heads because one has cut their head several times compacting into the floor below. The male on the revealing side also has an additional cut on his forehead that the first did not. Crawl space is tight especially for that guy. You can also see the shaking and speed change in the movement on both containers as they get in place. Still impressive timing to get it all done.

Here is a better video.

https://www.youtube.com/watch?v=bFxDNwPsIi0

You can definitely see both boxes shift once the curtain falls and they shift in such a way that it appears weight is being distributed from one end of the box to the middle.

still have no idea how he did it.

I have no idea, but there is serious movement going on in that box once the curtain falls.