The Power comes from an integrated battery that's supposed to double your run time. While I haven't needed the extra run-time, it was the only grip I've seen to date which is why I picked it up. CTA also has Hand Grip with Stand as well as Grip with Silicone Handles, I'd probably have gone with the Silicone model if they'd had them at Fry's.
The hand grip part works quite well, I don't get cramps anymore.
I thought the power support was broken initially. I had charged it up, and turned it on. It supplied power for about 2 seconds, then cut off for a couple seconds. It then turned back on and endlessly repeated the on/off cycle. You wouldn't think it'd matter, but the 3DS increases screen brightness when it's getting external power. The constantly changing screen brightness was annoying.
I took it back to Fry's and picked up another one. Charged it up, and had the exact same problem. Googled about it and this in a review at Amazon:
EDIT2: It seems that I might be coming to a conclusion on why my unit has the horrible screen flicker. If I have the unit and the 3DS fully charged then try to use the grip battery, I'll get the screen flicker. If I let the 3DS' charge go down some then use the grip battery, the connection stays solid with no flicker.
Sure enough, I turned off the grip then went into eShop and selected all the programs with updates to download. I then set it aside and watched TV while the downloads commenced. Later I was able to turn on the grip and there was no flicker at all.