[Mike Simonds]

What JR said about the halo is very useful.

I would also say that you need to find the point of attack and focus your energies there.

For example, sweep to your side, you want to look at the blocks in front of and to the sides of the runner. The trailing official will cover the blocks and other action behind the runner.

If I'm on the line as the H or L then I read the play as follows: snap, tackle, back. The most important read at the snap is the action of the offensive tackle closest to you. You need to read his initial action and determine if the play is a run or a pass. If pass then you pick-up your eligible receivers and related defenders. If run you look for clues where the point of attack will be: for example, as the umpire I'll follow the guard if he pulls or if there is a double-team block by the guard and the tackle then the run will probably occur there.

So in a nutshell: once you determine the point of attack, then officiate watching the halo around the ball. Scan the halo area, then focus on the individual blocks and other action, then react (no-call or flag), if no-call, then go back to scanning as you adjust your position to get the best look at your halo area.