Both cabin roof and the panel above entryway are not mounted rigidly, but free to move upwards. Such movement triggers emergency stop.

Edit: It's quite visible on this image if you know what to look for: http://en.wikipedia.org/wiki/Paternoster#/media/File:Paterno... Notice the (large-ish) "micro-switch" in front of the perforated panel on the right and quite large gap between cabin roof and paneling between cabins on the left.