When you press the button, a few lights should come on (there are a total of 5 lights). Like Techie said, they act like a quick way to gauge your battery level
I love that feature. It's kind of hidden, but cool.
<Captain Picard>THERE ARE FIVE LIGHTS!!!</Captain Picard>
It's hidden for me, I have a plastic case in the way. All I can do it look at it longingly.