But that isn't manipulable by someone standing in line, so it is effectively random. You can't determine exactly when the tsa person will press the button.
If that was the only requirement the TSA agent could direct you however they pleased as long as you couldn't predict it. The reason to use a software solution, one imagines, is to protect from corrupt TSA agents. Corrupt TSA agents know when they turn their iPads on.
Additionally, by observing several passengers being directed this way or that, you could brute force the seed and the system becomes deterministic.
Additionally, by observing several passengers being directed this way or that, you could brute force the seed and the system becomes deterministic.