Well, I'm not sure what signaling TECHNICALLY is, but I know this from readings what pros say about drafting.
Judging by what type of cards are being passed to you in the first couple of picks, you can determine by what colors are and aren't available which colors the person to your left is taking. Then, if you remember how what you immediately passed to your right, when the second pack comes, you can pretty much guess by what kinds of cards are not present what the player to your right has taken. It's complicated, but pros understand it.
Another thing that may be is that some players may actually have discreet signals to other players telling them what not to draft and what to draft. It may be as discreet as putting the cards in a certain order when you pass them to your buddy on the right to let him know what not to take in the second pack.
I don't approve of either method, but it is practiced.
Ransac, cpa trash man