I believe alot of crews use the traveling signal to let everyone know the ball is live. Ours does.

We do this as well.

Exactly when do they give it?

When the team is setting up for the FG as we are counting players.

In my association we use the "traveling" signal, but only between the ref and ump and it is to remind us that the snapper is protected, we do so any time a team is in scrimmage kick formation and thus the snapper would be protected.
Wing officials and back judges (we use 5 man as a max) do no give this signal.

Jasper

PS According to the new rule change we will probably only use it on situations where the snapper is still protected, I don't know if that portion of the rule has changed.

This is also how our crew uses it, the wings and BJ just notice it, we do not signal.

It's done discretely between officials when it is obvious a FG will be attempted.

Our signal is to grab and hold our whistle in our fist. We hold it out from our chest a few inches. That reminds everyone that it's a live ball and to lay off the whistle.

Intra-Crew Communication on Scoring Kicks
 

Not in the book, but...

Anytime Scrimmage Kick Formation is in use (snapper protected) R and U signal each other with travelling signal just as players are coming to their positions. (Before that its not a scrimmage kick formation)

Between downs when a wing observing substitutions on his sideline sees a kicking tee come in he makes the letter T with 2 fingers in front of his chest, while making eye contact with crew-mates. This signals the crew to get in position for a scoring kick, proper wing move to post, etc.

If it is not a try R pulls his whistle (we don't use finger whistles) in his fist out from his chest. HOLD THE WHISTLE.

We used to have a U who had been around for a long time. Kick tries were the 1 time the game he blew his whistle because when he went to a clinic in 1947 (year approximate) the mechanic was to have the U blow the whistle as soon as the kick try past his head, and no matter how many times we told him to stop he kept it up. He would regularly, 40% of the time, blow field goals dead. He retired before last season. (Thank goodness)

We give the rolling signal for 2 reasons...one to remind ourselves the ball remains live and the other (only on 4th down but this is when most scrimmage kicks occur) as a reminder the clock will stop after the play regardless of what happens.

As white hat I remind the others that this is a field goal as everyone is lining up. This comes as a result of the worst single IW in my career. Our then white hat blew an IW on a blocked FG attempt thinking it was a try. The ball turned into a greased pig with everyone whiffing on its recovery after the block. What made it even worse is that it took a few seconds for him to process what happened so that the play had time to progress before he tweeted. We replayed the down and the next attempt scored.

REPLY: Yes...we use the rolling hands (false start) signal as soon as we see the tee on the field and begin to move to our positions. And we make sure that all five of us see and echo it. If an official hasn't echoed to someone, we'll even yell to him and signal. As the BJ, the wing official under the post and I will remind each other "Ball's live" just before the snap.

We roll the hands also. I knew the answer and it appears that most do the same thing.

REPLY: And there's no real need to be discrete about it. Make sure that all 4, 5, 6, or 7 of you know that the ball's going to remain live if blocked or if it doesn't reach the endzone. And if you need to yell to your partner to remind him, do it.

When I said discrete I meant that we don't make a production of it we just signal amongst ourselves. Some astute coaches have picked up on some of our signals and use them accordingly. When a wing has punched a pass backward I have heard coaches yelling that it was a live ball. Some also know our counting signals and move people up on the line accordingly.