It's a circle cutter that acts a little like your school protractor used to - there's a graduated slide a pivot point at one end and a moveable small rotary cutter at the other end. I haven't used it yet but understand that you slide the little knob along the arm until you reach the desired radius for your circle and then screw it on to secure the cutter at that length. You then place the pivot point in the centre of your fabric and spin the arm around, cutting using the rotary cutter on the end of the arm.
Although I have a couple of other tools for cutting circles, this one appealed to me because of the flexibility in radius of circles you can cut.
I have an EZ Quilting shape cut circle ruler and small rotary cutter which work well but they only cut circles which have a diameter which is a whole number of inches - the smallest is 2", the largest 10" I think.
I also have the Olfa circular rotary cutter.
I'm hoping that the new tool will combine the best bits of all the above. Let's hope it doesn't combine the worst features as well!