The pair of flickering LEDs - red and white - indicate north and
south poles respectively - note that, as the magnet approaches,
they stop flickering before there is sufficient magnetic field
strength to trigger the numeric display. In marginal magnet
positions, the bias of flickering swings towards the appropriate
pole.
The meter also has a peak hold function - a single button toggles
between this and real-time. In peak hold mode, a green LED is lit.
The peak hold is set to 'lock' into the first pole it sees - a
higher value of the opposite pole won't trigger the meter. In the video,
a value from the magnet's north pole is held before the magnet is flipped
to present its south pole. The display continues to show the stored north
value until the meter is switched back to real time mode, whereupon the
current south pole is immediately indicated.
If the sensor is changed, the meter can be recalibrated by ensuring
that the sensor is clear of magnetic fields and pressing a button. This
stores an offset value based on the raw ADC reading, such that the new
device's null voltage is taken as zero. You can see the effect of
changing between calibrated ('CAL SEt') and raw ('CAL OFF') modes.
The device in the video is a few gauss south of the default calibration.
(The default is set to the middle of the ADC's 10-bit range of 0-1023.)
The meter stores its calibration setting in flash memory built into the
PIC - note that the flickering LEDs are resumed after the meter is switched
off and back on, rather than defaulting to the '8 gauss south' bias of this
particular sensor.