[spike021]

Not sure. Batters already look back to the dugout sometimes in order to know if they want a bunt, squeeze play, hit and run, etc. That doesn't take more than 3-5 seconds. If they already coordinate some kind of sign between the trainer (in question) and a player, then it shouldn't take any more time.

[fcbrooklyn]

The catcher throws the sign after the batter has taken his stance. No more looking at the dugout.

[bluGill]

3 seconds is more than enough time for the pitcher to realize the batter isn't paying attention and throw a strike. Batters will look of course, but they step out of the batters box (or otherwise - I don't know all the rules) to indicate that a pitch cannot be thrown now.

Once the batter is ready to receive a pitch the catcher can signal a new pitch.