Vary the potentiometer and watch the phase angle between input and output change from 0 to 180deg. The input signal is first split (see voltage at collector and emitter), then recombined at the output node. The potentiometer varies the proportion of pure resistance to reactance (from the capacitor), resulting in varying amounts of phase shift.
Since the output mixing stage is an RC filter, phase shift changes with input frequency - a typically undesirable effect.
