This circuit offers an example of Wien bridge sinusoidal oscillator. Two diodes are used in the negative feedback loop in order to prevent immediate saturation activity of the OpAmp as a consequence of the Barkhausen condition.
By modifying the amplitude of the input signal at the non-inverting OpAmp terminal so as to cause a tension drop across the diodes greater than the diode threshold voltage - usually 0.6 V - those switch, in turn, to conduction and therefore bypass the resistor they're across. This, of course, modifies the (negative feedback-) closed loop gain, Vo/V-, which is also affected by the trimmer simmetry, as it could be seen by writing the gain expression for the non-inverting amplifier.
