Here's the Pokemon Go Plus website
Spinning Pokestops without holding your phone is a nice feature but it takes the fun out of the game if you don't know what Pokemon you are catching and there is no skill in throwing a ball. I assume you will still need the app open on your phone but at least you can have it in Battery Saver mode to save some juice. This seems mostly suited for spinning a ton of pokestops like if you were in a car or walking to work, I'm not sure that is worth $45