Having my hand on the master is something I don't do and don't feel is necessary, also, here's a good example why you want to let the FADEC finish the auto-start process before securing the master, not too long ago, one of our crews immediately turned off the master during a start fault.
A second attempt to start the engine produced a flame out of the front of the engine, nearly as big as the flame out the back of the engine, the engine which was new, had to be replaced.
D.L.