NFHS Officials Signal #2?

[officialtony]

What is the purpose of the new signal #2 which is said to indicate a line violation?
I believe this is the signal to use when a fault is occurred at the time of service.
Is it also the signal to use for a center line violation by a player?
Anyone know why the change in NFHS?

What signal do you then use to signal a ball that has crossed the centerline UNDER the net?

Thanks in advance.