The inner sheet and the outer sheet are separated by a distance full of water. The expansion due to steam pressure is exaggerated by the radius difference between
the inner and outer corners. The inner moves a little, the outer moves more because of being at a larger radius. The stay bends a little to take up the difference but
it's only capable of doing that a finite number of times before the breaks. The socket on the flex stay allows the stay to "revolve" with the radius change. So when you
do all fixed stays you pays your dues and takes your chances.