The pointing is the center declaring the Mike or making the calls for the protection. In most cases it’s the center making the line calls and the QB declaring the Mike. Last second adjustments need to be made because defenses will disguise blitz and coverages.
It could also be a cue for the snap during a silent count. It could be the guard tapping the center, the qb lifting his leg and it could be a hand signal. There are so many ways to do this and it could vary from week to week.
When you get Mossed its always memorable