The device has an accelerometer in it which measures the rate of acceleration.
From that, and using the equations of motion 0-60 is a doddle to work out.
v = u + at is one equation with
v = final velocity
u = initial velocity
a = acceleration
t = time
Now, we know the values for v (60mph) and u (0), the device can work out a (how accurately though is another matter) so you just rearrange the equation to make t the subject
Knew that A level in physics would pay off, after all these years....
Oh, I have one as well and it seems pretty accurate for 0-60 and qtr mile times.