I forgot to say, sorry, that we turned off the screen lock in the phone settings.If the screen is dark, you have to press the power button to turn it on. Otherwise you don't have to specifically open the app, you just put it on your arm:
Put the middle of the back of the phone (this is where the NFC is) onto the sensor on your arm and it beeps twice to tell you it's done the reading, which then shows instantly on your phone, along with a recent graph x
If the screen lock is on then you have to also swipe or put a pattern in or whatever first to open the phone before it will scan.