Ambient Light Sensor and auto brightness

I don't actually have an iPhone yet but I read in an article somewhere (maybe David Pogues review) in the past couple of days that the ambient light sensor isn't constantly adjusting when the phone is active. It only takes an ambient light reading each time the phone is awakened. Wherever I read that it gave the tip of holding your hand over the sensor to simulate a darker ambient setting and have a brighter screen, or conversely, to hold it in bright light when waking it to get a dimmer screen. But like I said, I just read that somewhere, I don't actually have one yet to test this out on.