Personal Trainer: Walking comes with two activity meters that track your day-to-day movements, but the tiny device is not without its quirks.
The activity meter is a small digital pedometer of sorts. The device has no power button, instead turning on or off when it detects movement. It has a built in infrared sensor for communicating with your DS and an LED that can give you some basic information when you're on the go.
When you shake the device the LED will light up. One red fleash means you have not yet achieved your step goal, two gree flashes means you have.
While it's a pretty compact little device that seems to do the job well, there's a laundry list of things that will screw up its count, according to the manual:
Walking in a crowd or other situation where you stop and start repeatedly. If the meter is loose inside a bag or held or attached loosely by a strap. If the movement is different from walking, like jogging, running or other vigorous activity. If the movement is softened by shuffling, walking barefoot or wearing slippers or sandals. If the meter is horizontal. If the meter is used in a very humid environment.
I suppose that's why it's called Personal Trainer: Walking and not Jogging.