On Wed, 12 Dec 2012 15:16:14 -0800, "David Farber"
wrote:

The reason I know it's supposed to move up and down is because the pin on
the other, functional set of doors does move freely up and down. There's a
spring inside to push it up with some moderate force.

The spring allows you to compress the pivot pin when you install the
door and seat it in the bracket / track. The roller is likely the same
method.