It can be used for more than one type of effect, but Michael is dead-on. What the reviews and ads refer to is using it as a way of keying in on which is your force bit of info in a one-ahead before you unfold the billet, one possible use other than as the key for a pseudo-psychometry type bit.
I can see how those ads may be a bit confusing, which is why Andi and my ads for it are written a bit differently on that point.
What you get are three pens that you can instantly tell apart in the natural handling of them. You have participants fold up their billets and clip them to the pens to hand them back (be it as a matter of convenience, to avoid touching them prematurely, or whatever you like (or don't mention any reason, it's not really necessary)). In the action of removing the billet from the pen, you can immediately know who drew/wrote which, before you even open the paper.
Hope this helps,