Welcome, JerseyLJ, to our forum. I hope you've found it enjoyable over the years and hope you become an active poster. Congrats on donning the white hat...you may wish to change your handle to 'JerseyWH'

. As usual, Calhoun nailed your question. In your post you stated the foul occurred during the backward pass. IF it had occurred AFTER the backward pass had been completed it would be considered a running play (2-33-2) with the basic spot the end of the run.
This is merely one of the 4 W's that the flagging official should report to you before you send Mr. Umpire on his journey.You'll find that penalty enforcement will become your biggest challenge, not is it a 5,10, or 15; but where it is to be enforced from. Take your time - I've screwed up some over the years and it was always because I got caught up it the excitement of the game and hurried .
Don't be afraid to discuss with your crewmates as 5 heads are always better than one. Hopefully you have a veteran umpire who has been stepping off penalties for years and will stop a potential error. Lastly, stop and face the press box before giving the proper signals. If you give signals while walking, it may appear to some that you are doing a dance from a bygone era

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Good luck and keep us informed of your season.