Why are some fuel-injected engines started with the mixture at idle cut-off?
Often times fuel injected engines can be difficult to start with the mixture rich, particularly if run recently and still hot.  Following a priming procedure with the electric boost pump then cranking with the mixture at idle cutoff allows the engine to flush out fuel from the injector manifold until an optimal fuel air mixture is reached and the engine starts up.  Quickly moving the mixture to full rich at this point will stabilize the engine at idle with an appropriate throttle setting.