I experimented with them. They are like the handles on skirts and such. If you create a control prop for (parented to) the hand, for example, "moving" it around simply changes the dials for the hand -- kind of like a puppeteer's string. But it's really no different from dialing numbers into the actor itself. (But a whole lot easier to select in certain circumstances because you can place the control prop wherever you like -- say, 20 feet above the figure's head -- and it (the prop) doesn't actual change position in the preview.)

There may well be a lot more to it that I don't know about. But that's how it works if you just parent to an existing body part.

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.