What is "Flexible Space"

Hi all.

On the Customize Toolbar feature of Mail, just what is the "Flexible Space" icon? How does it work? To me, the way it should work is this:

In the Customize mode, you drop the tool where you want it and then, drag (cmd/ctrl/etc.) the left/right endpoints to suit the size you need.

Logically, tat's what it should do, right? If this is not it, just how does the "Flexible Space" tool work?