Very simple design where you adjust the width using an included allen wrench. Because it uses screw threads to connect the three pieces you can create a very secure, tight fit!
I have one myself and it works really well; I'm even considering using it to attach my phone to the handlebar on my bike! That's how confident I am of the grip on this device. Then there's the added bonus of that it will work with any phones or devices I will get in the future. Highly recommended.