Essentially, it's a collection of panels, stacked towards the top. Each can be expanded or collapsed on its own. When a header is expanded, the headers below it "stick" to its bottom. This is in contrast to the classic accordion, where the headers fall all the way to the bottom of the containing area. See some examples of a "classic" accordion here, and here.
In our system we found this control quite useful. It allows progressive disclosure, but always the user knows what is open, and what is available. We used it as a substitute for an accordion, where users tend to miss the headers lying on the bottom of its area.
Do you know whether this control is a common practice? Can you point to examples? Does it have a proper name?