-1It's their way of communicating when they can't say anything. The wires they wear sometimes make things obvious; it looks like they're talking to themselves and everyone stares. This way, it's easier. If they're all across the room from each other, all they have to do is move their hands, and the others know that they've got something.
The signs change with each assignment. If they stayed the same, it would make things as obvious as the wires to whoever they might be looking for. Things travel fast in a business like theirs. They started it about a year ago, and already, the department calls it 'pulling an SVU' when detectives from other units do it. So if people on the other side of the law knew about it, they wouldn't be surprised.
This assignment isn't really any different from the others. They've done it before. But this time, the signs are different. It's a dangerous game they're playing, but they don't have a choice. The case is a high profile one. They have lives to save and a job to do. They don't have time to waste. So they split up when they reach their destination, so they can cover more ground. It's the way things have always gone, but this time, the stakes are higher. If they screw it up, the city will come down on them. So they keep their eyes on each others' hands, careful not to make it clear what they're doing.
Hours pass. But still they have nothing. They're starting to give up on making their case tonight, but they can't leave until it's over. So they keep doing what they're doing, and they wait. And suddenly, they see it. No one else notices, because it means nothing to them, but it means everything to the four of them. And they move when they see it, that sign coming from the one who drew primary, because it means they've closed their case.
It means they've got their guy.