The underground belt provides a "magic" functionality.
Instead of describing I show you:
More explained: https://wiki.factorio.com/index.php?tit ... /More_uses
The issue, we are talking about is this:
![Image](https://forums.factorio.com/images/ext/039fe7fba042d05f6fa9d99167d9c244.png)
Taken from viewtopic.php?f=6&t=30245
The idea
![Image](https://forums.factorio.com/images/ext/b921055dd4b522a136035bf4ff9d436b.png)
Taken from viewtopic.php?f=66&t=32225&start=10#p206087 - thanks to Mendel.
Taken from viewtopic.php?f=6&t=30245&p=192085#p192085 - thanks to FasterJump