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Your meter looks identical to mine bar the color and the name.
That "dif" button stands for difference and not differential. In fact the manometer is always in differential mode.
The "dif" mode is used where you wish to find the difference between 2 readings.
An example would be if you hit the "diff" button at 80 km/hr then increased your speed to 100 km/hr then the reading on the manometer would be the difference in pressure for the 2 speeds.
So it is possibly best not to hit the diff button at start of test but rather if you do not have a zero reading whilst stationary then hitting the "hold" button for 2 seconds will zero the gauge.
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