In some firing squads there's only one bullet and the rest are blanks -- though if you've been in one, some say you can tell the difference between the bullet and the blank. The idea is to give a plausible deniability to those that follow the order: "It wasn't my gun that had the bullet."

If the method for delivering the nuclear order is routine and 99.99% benign (except for the 0.01% where it's not) and the order is checked only by machines, then no one would know except for the person that made it.

Further, to enforce compliance with routine behavior, one just needs to threaten the family of the subordinates. Or better yet, occasionally take one of your subordinates and make an example out of them, once every two years say. Send them and their family to labor camps never to be seen again.