"I'm the one whose supposed to say that. You could have just said 'code word'. I would have got it." And it was. Dean had set it up as their code word, if someone was in trouble. Mainly having a gun pointed at them. He'd set it up when they were younger. Or well, not that long ago for Sam.
If that didn't work. Dean could think of a few other things that would convince Sam that he was who he said he was. A bunch of them required him moving. So Dean really hoped it worked. Sam knew how to use a gun since he was nine. He knew where to aim to make it hurt.