By adjusting the yellow slider, the resistance of the variable resistor can be precisely tuned. Moving the top slider up results in a higher resistance, moving it down results in a lower resistance.
How does it work?
The variable resistor is a tube that kinks and unkinks by moving the slider up and down. When the slider is moved upwards, the two tubes are pulled towards each other, which results in a kink. This kink restricts the amount of air that can flow through the tube, which increases the resistance of the resistor.