matchless metal locator circuit working principle

[DJSUOL]

Sorry to bother you guys. I am preparing the presentation now and I need to figure out the working principle of 'matchless metal locator'. In my opinion, at the normal condition the Rx will receive a small signal and after the amplifier the signal voltage is still less than the 2/3Vcc so the ICB1 part will produce a high voltage. If a metal is near the coils, the received signal becomes larger and the amplified signal is larger than 2/3 Vcc so the chip could produce a square wave signal, am I right? Or the circuit works due to the change of frequency of received signal?

[DJSUOL]

[Carl-NC]

The first opinion is correct, though I'm not 100% sure about the threshold level. Sounds right, tho.