how are these hanging platforms standing up?

There’s a builder on Tweeter [sic] that made a raft with "floating" platforms and ziplines. He claims that there are no mods.

However, floating platforms should not be possible. The blocks should break if they have no support underneath them.

The image is here:

What I could see was:

               | <---- zipline post
              -- <---- wooden floor with something "round and spiky" underneath?
             |=|
             |=| <---- two support beams and a ladder
           |      <--- lower zipline post
          -- -- <------ two wooden floors with something "round and spiky" underneath?
             * <---- some building block that's under all platforms. I guess that's what is deceiving the game into thinking the platform should stand.

I believe the thing underneath the platforms (maybe a pipe?) is the key to making them float. What is it, and HOW can i build it? Is there any temporary scaffolding necessary?