How does it work?

"The city stream is redirected in an underground, invisible loop so that it flows backward, or upstream, for a short distance. However, the backflow is always just outflow in reality. This is because the watercourse has a continuous gradient." (translated from a Google result excerpt of this paywalled article [0])

[0] https://www.derbund.ch/warum-der-stadtbach-aufwaerts-fliesst...)

Yes, that seems to be the explanation. I found a few pictures here: https://www.komoot.com/de-de/highlight/615007 but they also do not show how it works.