Touchid - press or don't press?

Got my iPhone 5S and am very confused by the touch ID.

From the looks of it, I need to press my home button having set finger-pass code lock to unlock the phone.

I was under the impression that I simply had to touch the ring without pressing on the home button to unlock the phone. If I had to press on it, what's the point of the ring? Surely everyone would be using a finger to unlock anyway? Why bother with a metal ring to sense finger?

Am I not setting my phone right?