Seems to be related to the fuel injection system.
https://www.pennula.de/luftfahrt-ges...42-heft-25.htm
No 18 in the image is labelled as "Überfettung und Stoppzug" so that may be related to the very rich fuel mix the Jumo used for take-off power. If they can't cut that off then it would burn fuel like hell and quickly overheat.
EDIT: could also be a system to stop fuel injection if the engine has to be shut down