In this circuit, there are two feedback paths around an op-amp. One is positive dc feedback which forms a Schmitt trigger. The other is a CR timing network. Imagine that the output voltage is +10 V. The voltage at the noninverting terminal is +15 V. The voltage at the inverting terminal is a rising voltage with a time constant of CTRT. When this voltage exceeds + 5 V, the op amp's output will go low and the Schmitt trigger action will make it snap into its negative state.
Now the output is -10 V and the voltage at the inverting terminal falls with the time constant as before. By changing this time constant with a variable resistor, a variable frequency oscillation may be produced.
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