This is similar to a Dutch Oven, except much, much, worse, and is a reference to a popular method of execution during the Holocaust.
As history tell us the German oven will contain Zyklon B an will therefore kill the fellow occupant of the bed.
A brutal joke with dark consequences, but it is German after all.
Anne: Okay, mein Fürher
*Anne gets in bed and begins to choke and slowly suffocates while Adolf watches on in admiration*
Then one locks the other participant (Preferably the one who did not do the poop) under the sheets.
Person2 "That's how a German Oven works"