:The RC produces 60 deg phase shift, so Supposing the output of the amp is 180 out of phase, three such RC networks would eventually produce zero phase shift or in phase oscillations for feedback, thus positive feedback in short.
2.
When that stimulus is suddenly removed, you often perceive the opposite stimulus ( so if you've been oscillating around on waves all day, when the oscillation stops, you perceive the anti-phase oscillation that your brain has been using to cancel the real oscillation ).
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