Positioning of the propeller relative to the crankshaft is arranged such that the engine pistons pass through top dead centre during the swinging stroke.
More generally, the dead centre is any position of a crank where the applied force is straight along its axis, meaning no turning force can be applied.
It has a hemi-head and, unusually, the spark is fired after top dead centre because of the very high flame speed in a compact engine with this head design.
Not only did this provide maximum tractive effort when starting but also avoided problems that might arise if the high pressure piston was in a dead centre position.
Therefore, if both cranksets could be at dead centre at the same time, and the wheels should happen to stop in this position, the locomotive could not be started moving.