Sometimes you'll have to adjust or check the reference voltage of the stepper drivers. To do this you'll need a digital multimeter set on the 2V DC range.
In the Velleman manual the suggested method is to use the tips of your multimeter probes on the (-) terminal of the power connector and the + on a small test point and then use your third hand (!) to adjust the potentiometer with a screwdriver.
This test point is awfully close from the pins of the IC and if you slip and short them you can kiss your driver goodbye..
A far safer way is to use a screwdriver with a metallic tip and crocodile clips to connect the screwdriver to the + probe of the voltmeter (and the - to the power connector) so you have your hands free to adjust the voltage..
Indeed, the potentiometer can be used as the test point (it's even the only test point on Pololu's purple replacement drivers)..
Voltage must be set between 0.89 and 0.925 V.