As can be seen in the photo below, a front view of the BMW 801 with the propeller removed shows some writing on the fan hub.
This says, "Motor nicht am Lufterrad durchdrehenl" Translated literally this says, "Engine not by the fanwheel turn".
This is an instruction to service and maintenance personnel not to hand-turn the engine using the cooling fan blades which, because of their magnesium alloy composition, were too brittle to allow this.
In some photos, the word, "Achtung!" meaning, "attention" precedes the instruction.