Prehistoric,
My opinion on this is that you don't move as such, but you obviously can't just lie there like a wet blanket either: you need to use effort and muscle tone to hold still and essentially provide resistance to her movement.
Typically I will hold her hips and use upper-body strength to hold her pelvis down against mine, maximizing the "grinding" effect while she provides the actual movement with her own efforts.
I don't tend to use this position much any more, since I can generally provide more pleasure on top of from behind, but it's useful if you're physically exhausted after giving her one climax after another and need a breather. Also it can be good psychologically to take a break from dominating and controlling her pleasure occasionally, and let her "get it for herself".
To that end, having her grab the headboard/rails above your head is effective. It is also somewhat easier to get her to squirt in this position in my experience, although it is totally possible to do it in missionary intercourse and indeed foreplay as well.
Marty