Butt talk in public transportation

On public transportation you are constantly trying to maintain some semblance of personal space, while sharing with the world. On trains where all of the seats are in long row benches along either side, the attempt to maintain personal space manifests itself by people leave small amounts of space on either side between them and others sitting on the bench. As new people enter, sit, and leave there is a constant shuffle and scooching from side to side in an attempt to maintain some level of space. What I find most interesting about this phenomenon is that there seems to have developed an unspoken code to indicate your intention to sit. I witnessed the perfect example of this code yesterday.
It was early, just before the evening rush, and the train was somewhat crowded with most of the seats filled. There was a woman on the bench across from me who had just enough room on either side of her that a child might have been able to sit there, but not an adult. Several people entered the train, looked to either side of the woman, determined they would not fit, and moved on, but one man looked to either side, chose the side to the woman’s left and indicated his intention to sit in that spot. Although the woman had ignored the other people once the indication to sit was evident, she moved to the right effectively combining the two spaces into one large enough for the man to sit in. What is this powerful indicator that has the power to move tired commuters? When you intend to sit in a space, you approach that space, then turn your backside to the people already sitting and jut out your buttocks. Much like in the animal kingdom, this presentation of the butt acts as a signal to those around you of your intentions. Whether it is actual comprehension of the intended signal or just fear of being sat upon, people move over and so this appears to be an effective means of communication, as let’s face it, no one wants to talk to you on the train.