Tim, the german manual I hinted you at says (#12): Taster für Temperaturkontrolle
Which means: Momentary Switch for Temperature-Control
From what the text says, I get the idea that you pre-set the temperature at the temperature dial, switch on the processor and by pushing that switch switch on the lamp inside the on-off switch. When the lamp does not switch on when pressing that momentary switch, the pre-selected temperature has been reached.
Only switch on the processor when filled with water.
You could type down the text of the manual and feed that to an automated translator. That should give some usable result.
That is right!!! You had to push the switch to check the glow status of the temperature indicator so it would not glow all the time and potentially fog the paper. It's use is intuitive, and I forgot all about it.