To create a input signal with frequency variation for the Capacitor to charge and discharge a JK flipflop (frequency divider) is used.
When resistance is kept at minimum, the Capacitor charges immediately.
During positive input cycle, the capacitor is fully charged, voltage across resistor becomes 0.
Next during the negative cycle, the capacitor acts as source, discharging current. Thus the resistor will read a negative voltage.
This is a High Pass filter.
