A few weeks ago I went to watch Mike ref a game. I didn't know anyone on either of the teams playing, so my full attention was on the refs. During this particular game Mike was a substitute for a different crew, so he was the umpire the whole game.
As umpire, Mike stood behind the defensive line and linebackers, to watch the blocks made by the offensive line and defenders trying to stop the blocks. He also looked for holding or illegal blocks. Prior to the snap of the ball, he counted to make sure the right number of offensive players were on the field.
As the umpire Mike stands where much of the play's initial action occurs, so he is considered to hold the most dangerous officiating position.
Mike was also in charge of placing the ball on the line of scrimmage and signally what down it was.
Since I wasn't particularly interested in the outcome of the game, I had some time to observe different aspect of the game that I wouldn't normally see. One thing that made me laugh was the large range in body sizes of guys on the team. Since this was a JV game, these boys were probably about the same age, but you wouldn't be able to tell by looking at them.