The output of an VCO feeds two VCA´s audio input. A sine waveform LFO is set up to modulate both VCA´s.

We have an inverter to reverse the polarity of the applied voltage, being inserted in the control line of one of the units

When the control voltage supply (LFO) is set to its maximum value VCA2 (down) will be set fully on, and VCA1 (up) fully off. As the voltage is reduced, the resultant fall in the output of VCA2 will be matched by a corresponding rise in the output of the VCA1, the former level falling to zero and the latter rising to a maximum at the other extreme.