Can someone explain this?

note the dropout which have some kind of 'teeth' on it, when you want to change gear, says to a spinner one, you simply adjust the levers that not only move the chain up, but also move the wheel forward a bit to keep the chain tension tight enough.

in addition if you want to take up slack then you have to pedal backwards. there is no chain tensioner at all.

campagnolo paris roubaix gear mechanism developed in 1932