John,

Unfortunately, the app cannot run by itself. You have to tap a button and hold the phone for 10 seconds. The app is calibrated to measure tremor frequency and amplitude. The tremor peaks tend to be between 5 and 10hz. The app sensors tremors in all directions (x,y,z) and calculates an overall magnitude amplitude in centimeters.

Its basic function is to first set a baseline and then it measures future readings against the baseline score. I have no idea how accurate the measures are, but it certainly works well for following trends versus the baseline.

For anyone interested, the app is free at this time
Lift Pulse