Twice I saw this yesterday, in two different games; I had never seen it before.

The referee used a mechanic that involved him "brushing" his chest downward with both arms, sort of like if you were sweeping lint off of your coat or suit. This was done 3 or 4 times, one right after another.

What was that?

If it was on a field goal or extra point, they were identifing who the eligble receivers (w/ ineligible numbers) were to the rest of the crew. I think they added the mechanic after the SF/NYG playoff game last year.

ocref is right. It's also when an ineligible reports to an eligible position in the NFL. Watch a game involving the Houston Texans and you'll see this on almost every down as they announce that "73 is an eligible receiver".

Gets annoying after about the 20th play.

The NFL has a rule that an ineligible number cannot line up on the end of the line (in an eligible position) so he must report or it is an illegal formation. Most of the time, its called not when a guy forgot to report but rather when the wideout isn't on the line.