When attached to the back of your handheld device, you can then slip your finger (or fingers if you are using the two-ringed version) into the HANDeBand's rings and securely hold your device without worrying about it slipping out of your hands. In fact, you don't even have to actually hold onto the device once it is on your finger(s).
The HANDeBand comes in two parts. One is a circular base that attaches to your device; I tried it primarily on my iPhone 4. This base connects to your mobile device with a strong adhesive layer. The other part is the section that your finger(s) slip into and they slide together rather easily and in such a way as to allow the base and ring to rotate so that you can position the mobile device at any angle on your finger.
The HANDeBand comes in three sizes and there is a size chart on the back of the packaging, but it wasn't until I explored the product's website that I learned how to tell which of the sizes I had in my possession. Based on the size chart saying I needed a medium HANDeBand and the fact that there were two dots on the HANDeBand itself, I seem to have gotten the recommended size for my fingers. I will say that it was a pretty tight fit at first, but after a couple of weeks, the ring loosened up some and felt like it was where it should be.
The packaging also claims that the HANDeBand can be used as a stand to lay your mobile device propped up at an angle for watching video, and while that is definitely possible, it feels more like an afterthought than anything else. Of course, given a device bigger than a phone, like an iPad for example, this feels far less practical.
So why do you apparently need the HANDeBand? It is supposed to reduce finger strain from lengthy mobile device use and allow you to text or e-mail faster. In both cases, the reason is because you don't have to actively hold your device and you can just let it rest in your hands without worrying about it slipping off.