"Mannin-densha" is a term that conveys that not only are all of the seats in a train taken, but indeed every 800cm square - or whatever the absolute minimum a standing body requires - is also taken. The last few souls to get in have to back in, simultaneously pushing with whatever strength the legs below, and arms above (pulling from the inner upper edge of the door frame) can muster, compressing the standing bodies within. It's not pleasant.
Now - on to the "mechanics" bit.... Some people try to get themselves some space by moving their feet into someones feet space either behind or in front of them. The stupid thing here is that when you've got people up against you on all sides, there is nowhere to go! So... if someone takes the space you could have used to resist (a little - there's only so much you can do) having to lean into such a would-be space-grabber. The result? You have you lean into them much harder! In a bid to grab space, they end up forcing you to lean into them harder. Every time it happens, I mentally shake my head and think "Mechanics... if only you understood mechanics, you would be space-thief you!".
You folks living in less crowded cities - enjoy your space! You don't know how lucky you are!
Lyle (Hiroshi) Saxon